Heat, Its Production And Transmission
Chapter VII. Heat, Its Production And Transmission.
A clear understanding of the several sources, effects, and modes of transferring heat is of importance to everyone living in our complex civilization, especially when we consider the multitudes of objects that have as their principal use the production, transfer or utilization of heat.
139. Principal Sources of Heat.?First and most important is the Sun, which is continually sending to us radiant energy in the form of light and heat waves. These warm the earth, make plants grow, evaporate water, besides producing many other important effects.
Second, chemical energy is often transformed into heat. One has but to think of the heat produced by burning coal, wood, oil, and gas, to recognize the importance of this source. Chemical energy is also the source of the heat produced within our bodies. The action of quicklime and water upon each other produces much heat. This action is sometimes employed during balloon trips as a means of warming things.
Third, Electrical Energy.?In many cities electric cars are heated by the electric current. We have all heard of[Pg 160] electric toasters and other devices for heating by electricity. Electric light is produced by the heating of some material to incandescence by an electric current. The electric furnace has a wide application in the preparation and refining of metals.
Fig. 120.?Boy-scout method of making fire by friction.
Fourth, heat is also produced whenever mechanical energy of motion is overcome, whether it be by friction, concussion, or compression. Friction always
results in the production of heat, as when we warm our hands by rubbing
them together. When friction is excessive, such as in the case of a
heavy bearing not properly oiled, the bearing may get very hot. This is
the cause of the "hot box" on a railway car. Friction may produce heat
enough to set wood on fire. Some fires in mills are believed to be due
to this cause. Every boy scout must learn how to produce fire by friction. (See Fig. 120.) Concussion
may be illustrated by the heating of a piece of metal by hammering it,
while the compression of a gas always makes it warmer, as those who have
used a bicycle pump have observed. The production of heat by
compressing a gas is illustrated by the "fire syringe" (Fig. 121). This
consists of a glass tube with a tightly fitted piston. A sudden
compression of the air contained may ignite a trace of carbon bisulfid
vapor.
Fig. 121.?A fire syringe.
The interior of the earth is hot, but its heat seldom gets to the surface except at hot springs and volcanoes.
[Pg 161]
140. The Effects of Heat.?There are five important changes produced by heat: (a) change of size, (b) change of temperature, (c) change of state, as the melting of ice or evaporating of water, (d) chemical change, as the charring of sugar when it is overheated, and (e) electrical change. This is illustrated by the production of an electric current, by the heating of the junction of two different metals. A thermo-electric generator (see Fig. 122) has been constructed upon this principle and works successfully.
Fig. 122.?A thermo-electric generator.
2. Four sources of heat.
3. Five effects of heat.
4. Examples of each.
5. Illustrations of transformation of energy which involve heat.
2. Which three of the effects of heat do you make most use of? Explain what use you make of each of these effects.
3. Which of the forms of energy can be transformed into heat? How in each case?
[Pg 162]
4. Into what other forms of energy may heat be transformed? Name the device or process used in each case.
5. What five different commodities are purchased by people in your neighborhood for the production of heat? Which of these costs least for the amount of heat furnished? Which is most expensive? How do you determine these answers?
6. Why do many people buy heat in an expensive form, as in using an electric toaster, when they can obtain it in a cheaper form by burning gas or coal?
7. How many of the five effects of heat have you observed outside of school?
142. Units of Heat and Temperature.?There are two common units for measuring heat: the Calorie and the British thermal unit. The calorie is the amount of heat required to raise the temperature of a gram of water one centigrade degree. The British thermal unit is[Pg 163] the amount of heat required to raise the temperature of one pound of water one Fahrenheit degree. One of the units plainly belongs to the metric system, the other to the English.
An instrument for measuring temperature is called a thermometer. Various scales are placed upon thermometers. The two thermometer scales most commonly used in this country are the Centigrade and the Fahrenheit. The Fahrenheit thermometer scale has the temperature of melting ice marked 32?. The boiling point or steam temperature of pure water under standard conditions of atmospheric pressure is marked 212? and the space between these two fixed points is divided into 180 parts.
The centigrade thermometer scale has the same fixed points marked 0 and 100 and the space between divided into 100 parts. (See Fig. 123.) The centigrade scale is the one used by scientists everywhere.
Fig. 123.?Comparison of centigrade and Fahrenheit scales.
143. Comparison of Thermometer Scales.?It is often necessary to express in centigrade degrees a temperature for which the Fahrenheit reading is given or vice versa.
Since there are 180 Fahrenheit degrees between the "fixed points" and
100 centigrade degrees, the Fahrenheit degrees are smaller than the
centigrade, or 1?F. = 5/9?C. and 1?C. = 9/5?F. One must also take into
account the fact that the melting point of ice on the Fahrenheit scale
is marked 32?. Hence the following rule: To change a Fahrenheit reading
to centigrade subtract 32[Pg 164]
and take 5/9 of the remainder, while to change centigrade to Fahrenheit
multiply the centigrade by 9/5 and add 32 to the product. These two
rules are expressed by the following formulas.
A temperature of -40?F. is also represented by -40?C., therefore to change a Fahrenheit reading into centigrade, we add 40 to the given reading, then divide by 1.8 after which subtract 40. To change from a centigrade to Fahrenheit reading the only difference in this method is to multiply by 1.8 or
Fig. 124?Comparison of absolute, centigrade and Fahrenheit scales.
144. The Absolute Scale of Temperature.?One often hears the
statement "as cold as ice." This expresses the incorrect idea that ice
cannot become colder than its freezing temperature. The fact is that ice
may be cooled below freezing down to the temperature of its
surroundings. If a piece of ice is placed where the temperature is below
the melting point, the ice, like any other solid, cools to the
temperature of the surrounding space. For example, a piece of ice out of
doors is at 10?F. when the air is at this temperature. It follows then,
that when ice has been cooled below the freezing[Pg 165]
temperature that heat is required to warm the ice up to its melting
point; or in other words that ice at its melting temperature possesses
some heat. The temperature at which absolutely no heat exists is called absolute zero. There has been devised an absolute scale of temperature. This scale is based upon the centigrade scale, i.e., with 100? between the two fixed points; the scale, however, extends down, below the centigrade zero, 273?, to what is called absolute zero.
It follows therefore that upon the absolute scale, the melting point of
ice, and the boiling point of water are 273? and 373? respectively.
(See Fig. 124.)
The means employed to find the location of absolute zero are of much interest. It has been observed that when heated a gas tends to expand. If a measured volume of air at 0?C. is cooled or heated 1?C., it changes its volume 1/273, the pressure remaining the same. If it is cooled 10? it loses 10/273, if cooled 100? it loses 100/273 and so on. No matter how far it is cooled the same rate of reduction continues as long as it remains in the gaseous state. From these facts it is concluded that if the cooling could be carried down 273? that the volume would be reduced 273/273 or that the volume of the gas would be reduced to nothing. This is believed to mean that the molecular motion constituting heat would cease rather than that the matter composing the gas would disappear. Scientists have been able to obtain temperatures of extreme cold far down on the absolute scale. Liquid air has a temperature of -292?F., or -180?C. or 93?A. The lowest temperature thus far reported is 1.7?A. or -271.3?C., obtained in 1911, by evaporating liquid helium.
145. The Law of Charles.?The facts given in the last paragraph mean that if 273 ccm. of a gas at 0?C. or 273? A. are cooled 100?, or to -100?C., or 173?A., then it[Pg 166] will lose 100/273 of its volume or have a volume of 173 ccm. If warmed 100?, or up to 100?C., or 373?A., it will have a volume of 373 ccm. It follows then that in every case the volume will correspond to its absolute temperature, providing the pressure remains unchanged. The expression of this fact in scientific language is called the law of Charles. At a constant pressure the volume of a given mass of gas is proportional to its absolute temperature.
Expressed mathematically, we have V1/V2 = T1/T2. Compare the statement and mathematical expression of the laws of Charles and Boyle.
The formulas for the laws of Boyle and Charles are sometimes combined into one expression as follows:
2. Three thermometer scales, fixed points on each.
3. Absolute zero, how determined. Its value on each scale.
4. Law of Charles, its meaning. Combination of laws of Boyle and Charles.
2. A comfortable room temperature is 68?F. What is this temperature on the centigrade and absolute scales?
3. Change a temperature of 15?C. to F.; 15?F. to C.; -4?C. to F.; -20?F. to C.
[Pg 167]
4. The temperature of the human body is 98.6?F. What is this temperature on the absolute and centigrade scales?
5. The temperature of liquid air is -180?C. What is it on the Fahrenheit scale?
6. Mercury is a solid at -40?F. What is this on the centigrade scale?
7. How much heat will be required to raise the temperature of 8 lbs. of water 32?F.; 5 lbs. 10?F.?
Fig. 125.?A clinical thermometer used to take the temperature of the body.
8. How much heat will be required to raise the temperature of 30 g. of water 43?C.; 20 g., 50?C.?
9. Compute the temperature of absolute zero on the Fahrenheit scale.
10. Take three basins of water, one hot, one cold, and one lukewarm. If one hand be placed in the hot water while the other is placed in the cold and after a few minutes both are placed in the lukewarm water, this water will feel cool to one hand and warm to the other. Explain.
11. If 200 ccm. of air at 200? absolute is heated to 300?A. under constant pressure, what volume will the air occupy at the latter temperature?
12. How does one change a reading on the centigrade scale to a corresponding reading on the absolute scale?
147. Expansion of Liquids.?The expansion of liquids differs from that of gases in several important respects:
(a) Liquids have a smaller rate of expansion than gases. The rate of expansion per degree is called the Coefficient of Expansion. For example, the coefficient of expansion of a gas under constant pressure at 0?C. is {1/273} of its volume per degree centigrade.
(b) Different liquids expand at wholly different rates, that is, their coefficients of expansion differ widely. For example, the coefficient of expansion of mercury is 0.00018 per degree centigrade, of glycerine 0.0005 per degree centigrade, of petroleum 0.0009 per degree centigrade.
Fig. 126.?Gas thermometer.
(c) The same liquid often has different coefficients of expansion
at different temperatures. Water between 5?C. and 6?C. has a coefficient
expansion of 0.00002 per degree centigrade, between 8? and 50? of
0.0006, between 99? and 100? of 0.00076. The coefficient of expansion of
mercury, however, is constant for a wide range of temperature and,
therefore, it is well adapted for use in thermometers.
148. Peculiarity in the Expansion of Water.?Water has a peculiar rate of expansion. This is illustrated by the following experiment:
[Pg 169]
Fig. 127.?Apparatus used in testing the expansion of water.
149. The Expansion Of Solids.?Most solids when heated expand less than liquids and gases. Careful experiments show that expansion is:
(a) Proportional to the change in temperature.
(b) Different in different solids.
Here are a few coefficients of linear (length) expansion.
[Pg 170]
The coefficient of linear expansion is the fraction of its length that a body expands when heated one degree.
The coefficient of cubical expansion is the fraction of its volume that a body expands when heated one degree.
The expansion of solids is used or allowed for in many cases:
a. Joints between the rails on a railroad allow for the expansion of the rails in summer.
b. One end of a steel truss bridge is usually supported on rollers so that it can expand and contract with changing temperatures. (See Fig. 128.)
Fig. 128.?Truss bridge showing roller support at one end.
c. Suspension bridges have expansion joints where the ends of the
iron girders can move in or out of an expansion joint thus making the
bridge longer or shorter according to the temperature.
d. Iron tires are heated, slipped on to wagon wheels and then cooled, the contraction on cooling setting them tightly in place.
e. Metallic thermometers depend upon the movement due to the expansion of a coiled strip of metal which turns a pointer on the dial of the instrument. (See Fig. 129.)
[Pg 171]
f. The wires that are fused into glass in incandescent light bulbs must have the same coefficient of expansion as the glass. Platinum has therefore been used for this purpose. (See table above.)
Fig. 129.?Metallic thermometer.
2. Expansion of solids; peculiarities, applications.
3. Coefficient of linear expansion.
4. Coefficient of cubical expansion.
2. A man taking a full breath on the top of a mountain fourteen thousand feet high inhales 4 liters of air, the pressure being 40 cm. What volume would this same mass of air have in a place 600 ft. above sea-level when the barometer reads 75 cm. and the temperature is the same as on the mountain top?
3. If the coefficient of linear expansion of iron is 0.000012 per[Pg 172] degree C., how much will an iron bridge 1000 ft. long change in length in warming from -20?C. on a winter day to 30?C. upon a summer day.
4. What are some of the results that would follow in freezing weather if water continually contracted on being cooled to zero instead of beginning to expand when cooled below 4?C.?
5. Mention two instances that you have noticed of expansion occurring when a body is heated?
6. Compare the density of air at 30?C. with that at 10?C. at the same pressure. If both are present in a room, where will each be found? Why?
7. Compare the density of water at 40?C. with that at 10?C. If water at the two temperatures are in a tank, where will each be found? Why?
8. If water at 0?C. and at 4?C. are both in a tank, where will each be found? Why?
9. How much heat will be required to raise the temperature of a cubic foot of water 10?F.?
10. How much heat will be required to raise the temperature of 4 liters of water 25?C.?
11. How much longer would the cables of the Brooklyn suspension bridge be on a summer's day when the temperature is 30?C. than in winter at -20?C., the length of cable between the supports being about 1600 ft.
12. If 25 liters of air at -23?C. is warmed to 77?C. under constant pressure, what will be the resulting volume of air? Explain.
13. White pig iron melts at about 2000?F. Express this temperature upon the centigrade and absolute scales.
14. If 200 ccm. of air at 76 cm. pressure and 27?C. temperature be heated to 127?C. at a pressure of 38 cm. what will be the resulting volume?
15. A balloon contains 10,000 cu. ft. of gas at 75.2 cm. pressure and 24?C. It ascends until the pressure is 18 cm. and the temperature is -10?C. What is the volume of gas it then contains.
16. A gas holder contains 50 "cu. ft." of gas at a pressure of one atmosphere and 62?F. How much gas will it hold at 10 atmospheres and 32?F.
17. One thousand "cubic feet" of illuminating gas has what volume with 75 lbs. pressure and temperature of 10?C.
18. Define a "cubic foot" of illuminating gas.
[Pg 173]
150. Methods of Transmitting Heat.?One of the most practical benefits of the study of heat is clearer understanding of the different methods by which heat is transferred from one place to another and an intelligent idea of the means employed to prevent the transfer of heat.
It should be definitely understood at the beginning that cold signifies the absence of heat, just as darkness implies the absence of light, so when one speaks of cold getting into a house what is really meant is either the entrance of cold air by some opening or else the escape of the heat.
There are three distinct methods by which heat energy is transferred from one place to another, depending upon the medium or substance that transfers the heat.
a. A solid transmits heat by the method called conduction.
b. A fluid, either a liquid or a gas, transmits heat mainly by the method called convection.
c. Space transmits the energy of hot objects by the method called radiation.
Fig. 130.?Solids conduct heat.
151. Conduction.?To illustrate conduction, place in a gas
flame the ends of same metal wires supported as in Fig. 130. In a short
time the other ends of the wires become hot enough to burn one's hand.
This may be explained as follows: The hot gas flame contains molecules
in violent vibration and those striking the wire set its molecules
rapidly vibrating. Since, in a solid, the molecules are held in the same
relative positions, when one end of a wire is heated the rapidly
vibrating molecules at the hot end set their neighbors vibrating and
these the next in turn and so on until the[Pg 174]
whole wire is hot. It is a fortunate circumstance that different
substances have different rates of conductivity for heat. To realize
this, suppose that our clothing were as good a conductor as iron,
clothing would then be very uncomfortable both in hot and in cold
weather. The best conductors for heat are metals. It is interesting to
note that, as a rule good conductors of heat are also good conductors of
electricity, while poor conductors of heat are also poor electric
conductors. Careful experiments in testing the rate that heat will be
conducted through different substances show the following rates of
conductivity.
Fig. 131.?Water is a poor conductor of heat.
These figures are averages taken mainly from the Smithsonian Physical Tables:
To test the conductivity of liquids, take a test-tube nearly
full of cold water, hold the lower end in the hand while the tube is
inclined so that the upper end is heated by a gas flame until the water
boils. The lower end will be found to remain cold. (See Fig. 131.)
Careful measurements of the conductivity of water show that heat is
transmitted through it only {1/800} as rapidly as in silver, while air
conducts but {1/25} as rapidly as water.
[Pg 175]
Fig. 132.?Wall
construction of a refrigerator. 1, Porcelain enamel lining lock joint;
2, inside wood lining; 3, 3-ply red rope waterproof paper; 4, wool felt
deafening paper; 5, flaxlinum insulation; 6, dead air space; 7,
flaxlinum insulation; 8, wool felt deafening paper; 9, 3-ply red rope
waterproof paper; 10, outside wood case.
Fig. 133.?Sectional view of a Thermos bottle.
152. Non-conductors and Their Uses.?Many solids, however,
are poor conductors, as leather, fur, felt, and woolen cloth. These
substances owe their non-conductivity mainly to the fact that they are
porous. The air which fills the minute spaces of these substances is one
of the poorest conductors known and hinders the transfer of heat
through these solids. For the same reason loosely packed snow is a
protection to vegetation covered by it during a period of severe cold in
winter. The efficiency[Pg 176]
of storm sash or double windows, and of the double and triple walls of
ice-houses and refrigerators (see Fig. 132) in preventing the conduction
of heat is also largely due to the poor conductivity of the air
confined in the spaces between the walls. To prevent the circulation of
the air, sawdust, charcoal, and other porous material is often loosely
packed into the space between the walls of such structure.
Other illustrations of effective non-conductors will occur to every one; such as woolen clothing, wooden handles for hot objects, and the packing used in fireless cookers. A Thermos bottle is effective as a non-conductor of heat because the space between the double walls has the air exhausted from it (Figs. 133 and 134).
Of several objects in a cold room, some feel much colder to the touch than others, thus iron, marble, oil cloth, and earthenware will feel colder than woolen cloth, carpet, feathers, or paper. The first four objects feel cold because they are conductors, and conduct the heat away from the hand rapidly. The other substances named are non-conductors and hence remove heat from the hand less rapidly, and therefore do not feel so cold. In a similar way, if several hot objects are touched by the hand, the good conductors are the ones which will burn one most quickly by conducting heat rapidly to the hand. The non-conductors, however, will rarely burn one. Why are the handles of hot utensils often made of non-conducting materials such as wood, cloth, asbestos, etc.?
Fig. 134.?Cross-section of the vacuum flask in a Thermos bottle.
153. Radiation is the method by which heat comes to us from
the sun across space containing no tangible matter. It is also the
method by which heat gets to us when we[Pg 177]
stand near a fire. Everyone has noticed that this heat is cut off by
holding an object between the person and the fire. This fact indicates
that radiant heat travels in straight lines.
The radiation of heat is believed to be accomplished by means of waves in a medium called ether, which is invisible and yet pervades everything. Three of the most important characteristics of radiation are first, heat is transferred by radiation with the speed of light, or 186,000 miles per second. This fact is shown by the cutting off of both the sun's heat and light at the same instant during an eclipse of the sun. Second, radiant heat[I] travels in straight lines, while other modes of transferring heat may follow irregular paths. The straight line motion of radiant heat is shown by its being cut off where a screen is placed between the source of heat and the object sheltered. Third, radiant heat may pass through an object without heating it. This is shown by the coldness of the upper layers of the atmosphere and also by the fact that a pane of glass may not be heated appreciably by the heat and light from the sun which passes through it.
When radiant energy falls upon any object it may be (a) reflected at the surface of the object, (b) transmitted through the substance, (c), absorbed. All three of these effects occur in different degrees with different portions of the radiation. Well-polished surfaces are good reflectors. Rough and blackened surfaces are good absorbers. Transparent objects are those which transmit light well, but even they absorb some of the energy.
154. The Radiometer.?Radiant heat may be detected by means of the radiometer (Fig. 135). This consists of a glass bulb from which the air has been nearly exhausted.[Pg 178] Within it is a wheel with four vanes of mica or of aluminum mounted on a vertical axis. One side of each vane is covered with lampblack, the other being highly polished. when exposed to radiant heat from any source the vanes revolve with the bright side in advance.
The bulb is so nearly exhausted of air that a single molecule remaining may travel from the walls of the bulb to the vanes without coming in contact with another molecule.
The blackened sides absorb more heat than the highly polished sides. The air molecules striking these blackened sides receive more heat and so rebound with greater velocity than from the other side, thus exerting greater pressure. The blackened sides therefore are driven backward. If the air were not so rarified the air molecules would hit each other so frequently as to equalize the pressure and there would be no motion.
Fig. 135.?A radiometer.
Sun's Radiation.?Accurate tests of the amount of the sun's
radiation received upon a square centimeter of the earth's surface
perpendicular to the sun's rays were made at Mt. Wilson in 1913. The
average of 690 observations gave a value of 1.933 calories per minute.
These results indicate that the sun's radiation per square centimeter is
sufficient to warm 1 g. of water 1.933?C. each minute. Although the nature of radiation
is not discussed until Art. 408-411 in light, it should be said here
that all bodies are radiating heat waves at all temperatures, the heat
waves from cool bodies being much longer than those from hot bodies.
Glass allows the short luminous waves to pass through freely but the
longer heat waves from objects[Pg 179]
at the room temperature pass through with difficulty. This is the
reason why glass is used in the covering of greenhouses and hot beds.
Water also absorbs many of the longer heat waves. It is therefore used
in stereopticons to prevent delicate lantern slides from being injured
by overheating.
2. Non-conductors; uses, best non-conductors.
3. Radiation, three characteristics.
4. The sun's radiation, amount. The radiometer.
2. Why are plants often covered with paper on a night when frost is expected?
3. Will frost form in the fall of the year sooner on a wooden or a cement sidewalk? Why? On which does ice remain longer? Why?
4. Why in freezing ice-cream do we put the ice in a wooden pail and the cream in a tin one?
5. Is iron better than brick or porcelain as a material for stoves? Explain.
6. Which is better, a good or a poor conductor for keeping a body warm? for keeping a body cool?
7. Should the bottom of a teakettle be polished? Explain.
8. How are safes made fireproof?
9. Explain the principle of the Thermos bottle.
10. Explain why the coiled wire handles of some objects as stove-lid lifters, oven doors, etc., do not get hot.
When a liquid or a gas is heated in the manner just described, the heat is said to be transferred by convection. Thus the air in the lower part of a room may receive heat by conduction from a stove or radiator. As it expands on being warmed, it is pushed up by the colder denser air about it, which takes its place, thus creating a circulation of the air in the room. (See Fig. 137.) The heated currents of air give up their heat to the objects in the room as the circulation continues. These air currents may be observed readily by using the smoke from burning "touch paper" (unglazed paper that has been dipped into a solution of potassium nitrate ["saltpeter"] and dried).
Fig. 136.?Convection in a liquid.
156. Draft of a Chimney.?When a fire is started in a stove
or a furnace the air above the fire becomes heated, expands, and
therefore is less dense than it was before. This warm air and the heated
gases which are the products[Pg 181]
of the combustion of the fuel weigh less than an equal volume of the
colder air outside. Therefore they are pushed upward by a force equal to
the difference between their weight and the weight of an equal volume
of the colder air.
The chimney soon becomes filled with these heated gases. (See Fig. 138.) These are pushed upward by the pressure of the colder, denser air, because this colder air is pulled downward more strongly by the force of gravity than are the heated gases in the chimney.
Other things being equal, the taller the chimney, the greater the draft, because there is a greater difference between the weight of the gases inside and the weight of an equal volume of outside air.
Fig. 137.?Convection currents in a room.
Fig. 138.?Fire place showing draft of a chimney.
157. Convection Currents in Nature.?Winds are produced by differences in the pressure or density of the air, the movement being from places of high toward places of low pressure. One of the causes of a difference in density[Pg 182] of the air is a difference in temperature. This is illustrated by what are called the land and sea breezes
along the sea shore or large lakes. During the day, the temperature of
the land becomes higher than that of the sea. The air over the land
expands and being lighter is moved back and upward by the colder, denser
air from the sea or lake. This constitutes the sea breezes (Fig. 139). At night the land becomes cooler much sooner than the sea and the current is reversed causing the land breeze. (See Fig. 140.)
Fig. 139.?Sea breeze.
Fig. 140.?Land breeze.
The trade winds are convection currents moving toward the
hot equatorial belt from both the north and the south. In the hot belt
the air rises and the upper air flows back to the north and the south.
This region of ascending currents of air is a region of heavy rainfall,
since the saturated air rises to cool altitudes where its moisture is
condensed. The ocean currents are also convection currents. Their
motion is due to prevailing winds, differences in density due to
evaporation and freezing, and to the rotation of the earth, as well as
to changes in temperature.
158. The heating and ventilation of buildings and the problems connected therewith are matters of serious concern to all who live in winter in the temperate zone. Not only should the air in living rooms be comfortably heated, but it should be continually changed especially in the crowded rooms of public buildings, as those of schools, churches, and assembly halls, so that each person may be supplied with 30 or more cubic feet of fresh air per minute. In the colonial days, the open fire place afforded the ordinary[Pg 183] means for heating rooms. This heated the room mainly by radiation. It was wasteful as most of the heat passed up the chimney. This mode of heating secured ample ventilation. Fire places are sometimes built in modern homes as an aid to ventilation.
Benjamin Franklin seeing the waste of heat in the open fire places devised an iron box to contain the fire. This was placed in the room and provided heat by conduction, convection, and radiation. It was called Franklin's stove and in many forms is still commonly used. It saves a large part of the heat produced by burning the fuel and some ventilation is provided by its draft.
Fig. 141.?Heating and ventilating by means of a hot-air furnace.
159. Heating by Hot Air.?The presence of stoves in living
rooms of homes is accompanied by the annoyance of scattered fuel, dust,
ashes, smoke, etc. One attempt to remove this inconvenience led to
placing a large stove or fire box in the basement or cellar, surrounding
this with a jacket to provide a space for heating air which is then
conducted by pipes to the rooms above. This device is called the hot-air
furnace. (See Fig. 141.) The heated[Pg 184] air rises because it is pushed up by colder, denser air which enters through the cold-air pipes. The hot-air furnace
provides a good circulation of warm air and also ventilation, provided
some cold air is admitted to the furnace from the outside. One objection
to its use is that it may not heat a building evenly, one part being
very hot while another may be cool. To provide even and sufficient heat
throughout a large building, use is made of hot water or steam heating.
Fig. 142.?A hot-water system of heating.
[Pg 185]
[Pg 186]
160. Hot-water Heating.?In hot-water heating a furnace arranged for heating water is placed in the basement. (See Fig. 142.) Attached to the top of the heater are pipes leading to the radiators in the various rooms; other pipes connect the radiators to the bottom of the boiler. The heater, pipes, and radiators are all filled with water before the fire is started. When the water is warmed, it expands and is pushed up through the pipes by the colder water in the return pipe. The circulation continuing brings hot water to the radiator while the cooled water returns to the heater, the hot radiators heating the several rooms.
161. Steam Heating.?In steam heating a steam boiler is connected to radiators by pipes. (See Fig. 143.) The steam drives the air out of the pipes and radiators and serves as an efficient source of heat. Heating by steam is quicker than heating with hot water. It is therefore preferred where quick, efficient heating is required. Hot water is less intense and more economical in mild weather and is often used in private homes.
Fig. 144.?Heating by an indirect radiator with side-wall register.
162. Direct and Indirect Heating.?In heating by direct radiation
(Figs. 142, 143), the steam or hot-water radiators are placed in the
rooms to be heated. With direct radiation, ventilation must be provided
by special means, such as opening windows, doors, and ventilators.
Sometimes radiators are placed in a box or room in the basement. Air
from out of doors is then driven by a fan over and about the hot
radiators. The air thus heated[Pg 187] is conducted by pipes to the several rooms. This arrangement is called indirect heating.
(See Fig. 144.) The latter method, it may be observed, provides both
heat and ventilation, and hence is often used in schools, churches,
court houses, and stores. Since heated air, especially in cold weather,
has a low relative humidity some means of moistening the air of
living rooms should be provided. Air when too dry is injurious to the
health and also to furniture and wood work. The excessive drying of wood
and glue in a piece of furniture often causes it to fall apart.
Fig. 145.?An automatic air valve.
Fig. 146.?An automatic vacuum valve.
163. Vacuum Steam Heating.?In steam heating, air valves
(Fig. 145) are placed on the radiators to allow the air they contain to
escape when the steam is turned on. When all the air is driven out the
valve closes. Automatic vacuum valves (Fig. 146) are sometimes used.
When the fire is low and there is no steam pressure in the radiators the
pressure of the air closes the valve, making a partial vacuum[Pg 188]
inside. The boiling point of water falls as the pressure upon it is
reduced. As water will not boil under ordinary atmospheric pressure
until its temperature is 100?C. (212?F.), it follows that by the use of
vacuum systems, often called vapor systems, of steam heating, water will
be giving off hot vapor even after the fire has been banked for hours.
This results in a considerable saving of fuel.
Fig. 147.?Plenum hot-blast system with temperature regulation.
164. The Plenum System of Heating.?In the plenum system of
heating (see Fig. 147) fresh air is drawn through a window from outdoors
and goes first through tempering coils where the temperature is raised
to about 70?. The fan then forces some of the air through heating coils,
where it is reheated and raised to a much higher temperature, depending
upon the weather conditions. Both the hot and tempered air are kept
under pressure by the fan in the plenum room and are forced from this
room through galvanized iron ducts to the various rooms to be[Pg 189]
heated. The foul air is forced out of the room through vent ducts which
lead to the attic where it escapes through ventilators in the roof.
Fig. 148.?A thermostat. (Johnson System.)
A thermostat is placed in the tempered-air part of the plenum room
to maintain the proper temperature of the tempered air. This thermostat
operates the by-pass damper under the tempering coils, and sometimes the
valves on the coils. The mixing dampers at the base of the
galvanized-iron ducts are controlled by their respective room
thermostats. Attic-vent, fresh-air, and return-air dampers are under
pneumatic switch control. A humidifier can be provided readily for this
system. This system of heating is designed particularly for school
houses where adequate ventilation is a necessity.
165. The Thermostat.?One of the many examples of the expansion of metals is shown in one form of the thermostat (Fig. 148) in which two pieces of different metals[Pg 190] and of unequal rates of expansion, as brass and iron, are securely fastened together.
The thermostatic strip T moving inward and outward, as affected by the room temperature, varies the amount of air which can escape through the small port C. When the port C is completely closed (Fig. 148a) the full air pressure collects on the diaphragm B which forces down the main valve, letting the compressed air from the main pass through the chamber D into chamber E as the valve is forced off its seat. The air from chamber E then passes into the branch to operate the damper.
When port C is fully open (Fig. 148b) the air pressure on diaphragm B is relieved, the back pressure in E lifts up the diaphragm and the air from the branch escapes out through the hollow stem of the main valve, operating the damper in the opposite direction from that when C is closed.
2. Convection. Drafts of a chimney. Land and sea breezes.
3. Heating and ventilation of buildings.
(a) By hot air.
(b) Hot-water heating.
(c) Steam heating.
(d) Direct and indirect heating.
(e) Vacuum steam heating.
(f) The plenum system.
(g) The thermostat.
2. Name three natural convection currents.
3. Explain the draft of a chimney. What is it? Why does it occur?
[Pg 191]
4. Make a cross-section sketch of your living room and indicate the convection currents by which the room is heated. Explain the heating of the room.
5. Make a sketch showing how the water in the hot-water tank in the kitchen or laundry is heated. Explain your sketch, indicating convection currents.
6. Is it economical to keep stoves and radiators highly polished? Explain.
7. If you open the door between a warm and a cool room what will be the direction of the air currents at the top and at the bottom of the door? Explain.
8. If a hot-water heating system contains 100 cu. ft. of water how much heat will be required to raise its temperature 150?F.?
9. Why does a tall chimney give a better draft than a short one?
10. Explain how your school room is heated and ventilated.
11. Should a steam or hot-water radiator be placed near the floor or near the ceiling of a room? Why?
12. In a hot-water heating system an open tank connected with the pipes is placed in the attic or above the highest radiator. Explain its use.
167. Conditions for Saturation.?If a liquid is evaporating into a vacuum, the molecules on leaving find no opposition until they reach the limits of the vessel containing the vacuum. Evaporation under these conditions goes on with great rapidity and the space becomes saturated almost instantly. If, however, air be present at ordinary pressure, many of the ordinary water vapor molecules on leaving are struck and returned to the water by the air molecules directly above. Those escaping gradually work their way upward through the air. This explains why it is that our atmosphere is not often saturated even near large bodies of water, the retarding effect of the air upon the evaporation preventing more than the layers of air near the water surface becoming saturated.
Just as the amount of salt that can be held in solution in a liquid is lessened by cooling the solution (Art. 26), so the amount of water vapor that can be held in the air is lessened by lowering its temperature. If air not moist enough to be saturated with water vapor is cooled, it will, as the cooling continues, finally reach a temperature at which it will be saturated or will contain all the water vapor it can hold at this temperature. If the air be still further cooled some of the water vapor will condense and may form fog, dew, rain, snow, etc., the form it takes depending upon where and how the cooling takes place.
168. The Formation of Dew.?If the cooling of the atmosphere is at the surface of some cold object which lowers the temperature of the air below its saturation point, some of its moisture condenses and collects upon the[Pg 193] cold surface as dew. This may be noticed upon the surface of a pitcher of ice-water in summer. At night, the temperature of grass and other objects near or on the ground may fall much faster than that of the atmosphere owing to the radiation of heat from these objects. If the temperature falls below the saturation point, dew will be formed. This natural radiation is hindered when it is cloudy, therefore little dew forms on cloudy nights. Clear nights help radiation, therefore we have the most dew on nights when the sky is clear. If the temperature is below freezing, frost forms instead of dew.
169. Formation of Fog.?If the cooling at night is great enough to cool the body of air near the earth below the saturation temperature, then not only may dew be formed, but some moisture is condensed in the air itself, usually upon fine dust particles suspended in it. This constitutes a fog. If the cooling of the body of air takes place above the earth's surface as when a warm moist current of air enters a colder region, e.g., moves over the top of a cold mountain, or into the upper air, then as this air is cooled below its saturation point, condensation upon fine suspended dust particles takes place, and a cloud is formed. If much moisture is present in the cloud, the drops of water grow in size until they begin to fall and rain results; or if it is cold enough, instead of rain, snowflakes will be formed and fall. Sometimes whirling winds in severe thunderstorms carry the raindrops into colder and then warmer regions, alternately freezing and moistening the drops or bits of ice. It is in this way that hail is said to be formed.
170. The Dew Point.?The temperature to which air must be cooled to saturate it or the temperature at which condensation begins is called the dew point. This is often determined in the laboratory by partly filling a polished[Pg 194] metal vessel with water and cooling the water by adding ice until a thin film of moisture is formed upon the outer surface. The temperature of the surface when the moisture first forms is the dew point.
171. The Humidity of the Atmosphere.?After the dew point has been obtained, one may compute the relative humidity or degree of saturation of the atmosphere, from the table given below. This is defined as the ratio of the amount of water vapor present in the air to the amount that would be present if the air were saturated at the same temperature.
172. Wet and Dry Bulb Hygrometer.?A device for indicating the relative humidity of the air is called an[Pg 195] hygrometer. There are various forms. The wet and dry bulb hygrometer
is shown in Fig. 149. This device consists of two thermometers, one
with its bulb dry and exposed to the air, the other bulb being kept
continually moist by a wick dipping into a vessel of water. An
application of the principle of cooling by evaporation is made in this
instrument. Unless the air is saturated so that evaporation is
prevented, the wet-bulb thermometer shows a lower temperature, the
difference depending upon the amount of moisture in the air, or upon the
relative humidity. Most determinations of relative humidity are made
with this kind of instrument. It is necessary in order to make an
accurate determination, to fan or set the air in motion about the
thermometers for some time before reading them. The relative humidity is
then found by using tables giving the relative humidity that
corresponds to any reading of the thermometers.
Fig. 149.?Wet and dry bulb hygrometer.
Fig. 150.?A dial hygrometer.
2. Formation of dew, fog, rain, and snow.
3. Dew point, relative humidity.
4. Use of the dry- and wet-bulb hygrometer. Goose-quill hygrometer.
2. How does the white cloud of steam seen about a locomotive in cold weather differ from fog? Explain.
3. In cold weather is the relative humidity of air out of doors and indoors the same? Explain.
4. Compare the relative humidity of air in a desert and near the ocean.
5. Look up the derivation of the term "hygrometer." Give the use of the instrument.
6. Find the relative humidity of air at 20?C. if its dew point is at 10?C.
7. How may the relative humidity of the air in a home be increased?
8. What is the effect of high humidity in the summer upon human beings? How do you explain this?
9. Does dew fall? Explain how dew is formed?
10. In what respects is a cloud similar to a fog? In what respects different?
11. Why are icebergs frequently enveloped in fog?
12. Does dew form in the day time? Explain.
This indicates that fanning a dry body at the temperature of the air does not change its temperature. Fanning does increase evaporation by removing the air containing the evaporated molecules near the surface of the liquid so that unsaturated air is continually over the liquid. If a pint of water is placed in a bottle and another pint in a wide pan the latter will become dry much sooner because of the greater surface over which evaporation can take place. Application of this is made at salt works where the brine is spread out in shallow pans.
175. Rate of Evaporation.?The rate of evaporation is affected by several factors. These have been illustrated in the preceding paragraphs. To briefly summarize:
The rate of evaporation of a liquid is affected by?
(a) The nature of the liquid.
(b) The temperature of the liquid.
(c) The pressure upon its evaporating surface.
(d) The degree of saturation of the space into which the liquid is evaporating.
(e) The rate of circulation of air over its surface.
(f) The extent of surface exposed to evaporation.
176. Molecular Motion in Solids.?Evidence of molecular motion in liquids is given by expansion on heating, evaporation, and diffusion. Do any of these lines of evidence apply to solids? It is a fact of common experience that solids do become larger on heating. Spaces are left between the ends of rails on railroads so that when they expand in summer they will not distort the track. Iron tires are placed on wheels by heating them until they slip on easily. Then on cooling, the iron shrinks and presses the wheel tightly. Many common demonstrations of expansion are found in lecture rooms. The fact of the evaporation of a solid is often detected by noticing the odor of a substance. The odor of moth balls is[Pg 199] one example. Camphor also evaporates. Heated tin has a characteristic odor noted by many. Ice and snow disappear in winter even though the temperature is below freezing. Wet clothes, "freeze dry," that is, dry after freezing, by evaporation. A few crystals of iodine placed in a test-tube and gently heated form a vapor easily seen, even though none of the iodine melts. Where the vapor strikes the side of the tube, it condenses back to dark gray crystals of iodine. This change from solid directly to gas and back again without becoming liquid is called sublimation. A number of solids are purified by this process.
2. Effects of molecular motion in solids: (a) Expansion, (b) Evaporation, (c) Sublimation.
2. Give an illustration for each of the factors affecting evaporation.
3. Give an illustration for each of the three evidences of molecular motions in solids.
4. Since three-quarters of the earth's surface is covered with water, why is not the air constantly saturated?
5. If the air has the temperature of the body, will fanning the perfectly dry face cool one? Explain. Will the effect be the same if the face is moist? Explain.
6. What is the cause of "Cloud Capped" mountains?
7. Why does the exhaust steam from an engine appear to have so much greater volume on a cold day in winter than on a warm one in summer?
8. What causes an unfrozen pond or lake to "steam" on a very cold day in winter, or on a very cool morning in summer?
9. As the air on a mountain top settles down the sides to places of greater pressure, how will its temperature be affected? its relative humidity? Explain.
10. On our Pacific coast, moist winds blow from the west over the mountains. Where will it rain? Where be dry? Explain.
- (1) Sources and Effects of Heat
- (2) Temperature and Expansion
- (3) Expansion of Gases, Liquids and Solids
- (4) Modes of Transmitting Heat
- (5) Convection, Heating and Ventilation
- (6) The Moisture in the Air, Hygrometry
- (7) Evaporation
(1) Sources and Effects of Heat
138. Importance of the Study of Heat.?Heat is brought to our attention through the sensations of heat and cold. In winter, we warm our houses and prevent the escape of heat from them as much as possible. In summer we endeavor to keep our living rooms cool and our bodies from being overheated.A clear understanding of the several sources, effects, and modes of transferring heat is of importance to everyone living in our complex civilization, especially when we consider the multitudes of objects that have as their principal use the production, transfer or utilization of heat.
139. Principal Sources of Heat.?First and most important is the Sun, which is continually sending to us radiant energy in the form of light and heat waves. These warm the earth, make plants grow, evaporate water, besides producing many other important effects.
Second, chemical energy is often transformed into heat. One has but to think of the heat produced by burning coal, wood, oil, and gas, to recognize the importance of this source. Chemical energy is also the source of the heat produced within our bodies. The action of quicklime and water upon each other produces much heat. This action is sometimes employed during balloon trips as a means of warming things.
Third, Electrical Energy.?In many cities electric cars are heated by the electric current. We have all heard of[Pg 160] electric toasters and other devices for heating by electricity. Electric light is produced by the heating of some material to incandescence by an electric current. The electric furnace has a wide application in the preparation and refining of metals.
Fig. 120.?Boy-scout method of making fire by friction.
Fig. 121.?A fire syringe.
[Pg 161]
140. The Effects of Heat.?There are five important changes produced by heat: (a) change of size, (b) change of temperature, (c) change of state, as the melting of ice or evaporating of water, (d) chemical change, as the charring of sugar when it is overheated, and (e) electrical change. This is illustrated by the production of an electric current, by the heating of the junction of two different metals. A thermo-electric generator (see Fig. 122) has been constructed upon this principle and works successfully.
Fig. 122.?A thermo-electric generator.
Important Topics
1. Importance of a study of heat.2. Four sources of heat.
3. Five effects of heat.
4. Examples of each.
5. Illustrations of transformation of energy which involve heat.
Exercises
1. Write a list of the sources of heat in the order of their importance to you. State why each is important to you.2. Which three of the effects of heat do you make most use of? Explain what use you make of each of these effects.
3. Which of the forms of energy can be transformed into heat? How in each case?
[Pg 162]
4. Into what other forms of energy may heat be transformed? Name the device or process used in each case.
5. What five different commodities are purchased by people in your neighborhood for the production of heat? Which of these costs least for the amount of heat furnished? Which is most expensive? How do you determine these answers?
6. Why do many people buy heat in an expensive form, as in using an electric toaster, when they can obtain it in a cheaper form by burning gas or coal?
7. How many of the five effects of heat have you observed outside of school?
(2) Temperature and Expansion
141. Heat and Temperature.?We should now clearly distinguish between the terms, heat and temperature. Heat is a form of energy consisting of molecular motion. The temperature of a body is its degree of hotness. The amount of heat present in a body and its temperature are very different things. The temperature refers to the intensity of the heat in the body. A quart of water and a red hot iron ball may contain equal amounts of heat, although the ball has a much higher temperature than the water. A cup of boiling water will have the same temperature as a tank full of boiling water, but the tank will contain more heat. Every one knows that it will take longer to boil a kettle full of water than a cupful. A hot-water bag, holding 2 quarts of water will give off heat longer than a 1-quart bag, both being filled with water at the same temperature. To put it in another way, more work is done in heating a large amount of water, than a small amount through the same change of temperature.142. Units of Heat and Temperature.?There are two common units for measuring heat: the Calorie and the British thermal unit. The calorie is the amount of heat required to raise the temperature of a gram of water one centigrade degree. The British thermal unit is[Pg 163] the amount of heat required to raise the temperature of one pound of water one Fahrenheit degree. One of the units plainly belongs to the metric system, the other to the English.
An instrument for measuring temperature is called a thermometer. Various scales are placed upon thermometers. The two thermometer scales most commonly used in this country are the Centigrade and the Fahrenheit. The Fahrenheit thermometer scale has the temperature of melting ice marked 32?. The boiling point or steam temperature of pure water under standard conditions of atmospheric pressure is marked 212? and the space between these two fixed points is divided into 180 parts.
The centigrade thermometer scale has the same fixed points marked 0 and 100 and the space between divided into 100 parts. (See Fig. 123.) The centigrade scale is the one used by scientists everywhere.
Fig. 123.?Comparison of centigrade and Fahrenheit scales.
(F.? - 32)5/9 = C.?, 9C.?/5 + 32? = F.?
Another method of changing from one thermometric scale to another is as follows:A temperature of -40?F. is also represented by -40?C., therefore to change a Fahrenheit reading into centigrade, we add 40 to the given reading, then divide by 1.8 after which subtract 40. To change from a centigrade to Fahrenheit reading the only difference in this method is to multiply by 1.8 or
C. = (F. + 40)/1.8 - 40 and F. = 1.8(C. + 40) - 40.
Fig. 124?Comparison of absolute, centigrade and Fahrenheit scales.
The means employed to find the location of absolute zero are of much interest. It has been observed that when heated a gas tends to expand. If a measured volume of air at 0?C. is cooled or heated 1?C., it changes its volume 1/273, the pressure remaining the same. If it is cooled 10? it loses 10/273, if cooled 100? it loses 100/273 and so on. No matter how far it is cooled the same rate of reduction continues as long as it remains in the gaseous state. From these facts it is concluded that if the cooling could be carried down 273? that the volume would be reduced 273/273 or that the volume of the gas would be reduced to nothing. This is believed to mean that the molecular motion constituting heat would cease rather than that the matter composing the gas would disappear. Scientists have been able to obtain temperatures of extreme cold far down on the absolute scale. Liquid air has a temperature of -292?F., or -180?C. or 93?A. The lowest temperature thus far reported is 1.7?A. or -271.3?C., obtained in 1911, by evaporating liquid helium.
145. The Law of Charles.?The facts given in the last paragraph mean that if 273 ccm. of a gas at 0?C. or 273? A. are cooled 100?, or to -100?C., or 173?A., then it[Pg 166] will lose 100/273 of its volume or have a volume of 173 ccm. If warmed 100?, or up to 100?C., or 373?A., it will have a volume of 373 ccm. It follows then that in every case the volume will correspond to its absolute temperature, providing the pressure remains unchanged. The expression of this fact in scientific language is called the law of Charles. At a constant pressure the volume of a given mass of gas is proportional to its absolute temperature.
Expressed mathematically, we have V1/V2 = T1/T2. Compare the statement and mathematical expression of the laws of Charles and Boyle.
The formulas for the laws of Boyle and Charles are sometimes combined into one expression as follows:
PV/T = P?V?/T?
or the product of the volume and pressure of a constant mass of gas is proportional to its absolute temperature.Important Topics
1. Heat units; calorie, British thermal unit.2. Three thermometer scales, fixed points on each.
3. Absolute zero, how determined. Its value on each scale.
4. Law of Charles, its meaning. Combination of laws of Boyle and Charles.
Exercises
1. Does ice melt at the same temperature at which water freezes? Express the temperature of freezing water on the three thermometer scales.2. A comfortable room temperature is 68?F. What is this temperature on the centigrade and absolute scales?
3. Change a temperature of 15?C. to F.; 15?F. to C.; -4?C. to F.; -20?F. to C.
[Pg 167]
4. The temperature of the human body is 98.6?F. What is this temperature on the absolute and centigrade scales?
5. The temperature of liquid air is -180?C. What is it on the Fahrenheit scale?
6. Mercury is a solid at -40?F. What is this on the centigrade scale?
7. How much heat will be required to raise the temperature of 8 lbs. of water 32?F.; 5 lbs. 10?F.?
Fig. 125.?A clinical thermometer used to take the temperature of the body.
9. Compute the temperature of absolute zero on the Fahrenheit scale.
10. Take three basins of water, one hot, one cold, and one lukewarm. If one hand be placed in the hot water while the other is placed in the cold and after a few minutes both are placed in the lukewarm water, this water will feel cool to one hand and warm to the other. Explain.
11. If 200 ccm. of air at 200? absolute is heated to 300?A. under constant pressure, what volume will the air occupy at the latter temperature?
12. How does one change a reading on the centigrade scale to a corresponding reading on the absolute scale?
(3) Expansion of Liquids and Solids
146. Expansion of Gases.?The law of Charles is found to apply to all gases. That is, all gases change in volume in proportion to the change of temperature provided the pressure remains constant. It is for this reason that we have the gas thermometer (see Fig. 126) which gives in skillful hands more accurate temperature readings than the best mercurial thermometer. Galileo devised and used the first air thermometer which consisted of a hollow[Pg 168] bulb blown on a glass tube and inverted in a dish of water. (See Fig. 1.) The water thermometer consists of a glass bulb filled with water which rises into a tube attached to the bulb. One disadvantage of the water thermometer is its limited range since it cannot be used below 0? or above 100?. Why?147. Expansion of Liquids.?The expansion of liquids differs from that of gases in several important respects:
(a) Liquids have a smaller rate of expansion than gases. The rate of expansion per degree is called the Coefficient of Expansion. For example, the coefficient of expansion of a gas under constant pressure at 0?C. is {1/273} of its volume per degree centigrade.
(b) Different liquids expand at wholly different rates, that is, their coefficients of expansion differ widely. For example, the coefficient of expansion of mercury is 0.00018 per degree centigrade, of glycerine 0.0005 per degree centigrade, of petroleum 0.0009 per degree centigrade.
Fig. 126.?Gas thermometer.
148. Peculiarity in the Expansion of Water.?Water has a peculiar rate of expansion. This is illustrated by the following experiment:
[Pg 169]
A test-tube filled with cold water is closed by a stopper
containing a small glass tube, the water extending up into the small
tube. (See Fig. 127.) The test-tube is placed in a freezing mixture of
salt and ice contained in a tumbler. As the water cools, the level of
the water in the small tube at first sinks. But before the water freezes it rises again, showing that after the water cools to a certain temperature that expansion of the water occurs with further cooling.
Careful tests show that the water on cooling contracts until it
reaches 4?C. On cooling below this temperature it expands. For this
reason, when the water of a lake or river freezes, the coldest water is
at the surface. On account of this the ice forms at the top instead of
at the bottom. If water contracted as it cooled to the freezing
temperature the coldest water would be at the bottom. Freezing would
begin at the bottom instead of at the surface. Lakes and rivers would
freeze solid. In the summer only in shallow waters would all the ice
melt. The result would be that fish and other aquatic life would be
killed. Climate would be so changed that the earth might become
uninhabitable. Since water is densest at 4?C. all the water in a lake or
river, when it is covered with ice, is at 4?C. except that near the
surface.
Fig. 127.?Apparatus used in testing the expansion of water.
(a) Proportional to the change in temperature.
(b) Different in different solids.
Here are a few coefficients of linear (length) expansion.
[Pg 170]
| Brass | 0.000018 per degree C. |
| Glass | 0.000009 per degree C. |
| Ice | 0.000052 per degree C. |
| Iron | 0.000012 per degree C. |
| Platinum | 0.000009 per degree C. |
| Zinc | 0.000027 per degree C. |
The coefficient of cubical expansion is the fraction of its volume that a body expands when heated one degree.
The expansion of solids is used or allowed for in many cases:
a. Joints between the rails on a railroad allow for the expansion of the rails in summer.
b. One end of a steel truss bridge is usually supported on rollers so that it can expand and contract with changing temperatures. (See Fig. 128.)
Fig. 128.?Truss bridge showing roller support at one end.
d. Iron tires are heated, slipped on to wagon wheels and then cooled, the contraction on cooling setting them tightly in place.
e. Metallic thermometers depend upon the movement due to the expansion of a coiled strip of metal which turns a pointer on the dial of the instrument. (See Fig. 129.)
[Pg 171]
f. The wires that are fused into glass in incandescent light bulbs must have the same coefficient of expansion as the glass. Platinum has therefore been used for this purpose. (See table above.)
Fig. 129.?Metallic thermometer.
Important Topics
1. Expansion of Liquids; peculiarities. Anomalous expansion of water and its results.2. Expansion of solids; peculiarities, applications.
3. Coefficient of linear expansion.
4. Coefficient of cubical expansion.
Exercises
1. The gas within a partly inflated balloon has a volume of 1000 cu. ft. at a pressure of 74 cm., and a temperature of 15?C. What will be the volume of the gas when its pressure is 37 cm. and the temperature is -17?C.?2. A man taking a full breath on the top of a mountain fourteen thousand feet high inhales 4 liters of air, the pressure being 40 cm. What volume would this same mass of air have in a place 600 ft. above sea-level when the barometer reads 75 cm. and the temperature is the same as on the mountain top?
3. If the coefficient of linear expansion of iron is 0.000012 per[Pg 172] degree C., how much will an iron bridge 1000 ft. long change in length in warming from -20?C. on a winter day to 30?C. upon a summer day.
4. What are some of the results that would follow in freezing weather if water continually contracted on being cooled to zero instead of beginning to expand when cooled below 4?C.?
5. Mention two instances that you have noticed of expansion occurring when a body is heated?
6. Compare the density of air at 30?C. with that at 10?C. at the same pressure. If both are present in a room, where will each be found? Why?
7. Compare the density of water at 40?C. with that at 10?C. If water at the two temperatures are in a tank, where will each be found? Why?
8. If water at 0?C. and at 4?C. are both in a tank, where will each be found? Why?
9. How much heat will be required to raise the temperature of a cubic foot of water 10?F.?
10. How much heat will be required to raise the temperature of 4 liters of water 25?C.?
11. How much longer would the cables of the Brooklyn suspension bridge be on a summer's day when the temperature is 30?C. than in winter at -20?C., the length of cable between the supports being about 1600 ft.
12. If 25 liters of air at -23?C. is warmed to 77?C. under constant pressure, what will be the resulting volume of air? Explain.
13. White pig iron melts at about 2000?F. Express this temperature upon the centigrade and absolute scales.
14. If 200 ccm. of air at 76 cm. pressure and 27?C. temperature be heated to 127?C. at a pressure of 38 cm. what will be the resulting volume?
15. A balloon contains 10,000 cu. ft. of gas at 75.2 cm. pressure and 24?C. It ascends until the pressure is 18 cm. and the temperature is -10?C. What is the volume of gas it then contains.
16. A gas holder contains 50 "cu. ft." of gas at a pressure of one atmosphere and 62?F. How much gas will it hold at 10 atmospheres and 32?F.
17. One thousand "cubic feet" of illuminating gas has what volume with 75 lbs. pressure and temperature of 10?C.
18. Define a "cubic foot" of illuminating gas.
[Pg 173]
150. Methods of Transmitting Heat.?One of the most practical benefits of the study of heat is clearer understanding of the different methods by which heat is transferred from one place to another and an intelligent idea of the means employed to prevent the transfer of heat.
It should be definitely understood at the beginning that cold signifies the absence of heat, just as darkness implies the absence of light, so when one speaks of cold getting into a house what is really meant is either the entrance of cold air by some opening or else the escape of the heat.
There are three distinct methods by which heat energy is transferred from one place to another, depending upon the medium or substance that transfers the heat.
a. A solid transmits heat by the method called conduction.
b. A fluid, either a liquid or a gas, transmits heat mainly by the method called convection.
c. Space transmits the energy of hot objects by the method called radiation.
Fig. 130.?Solids conduct heat.
Fig. 131.?Water is a poor conductor of heat.
| Silver | 100 |
| Copper | 74 |
| Aluminum | 35 |
| Brass | 27 |
| Zinc | 26 |
| Iron | 15 |
| Tin | 14.7 |
| German silver | 8.4 |
| Mercury | 1.7 |
| Granite | 0.53 |
| Limestone | 0.52 |
| Ice | 0.5 |
| Glass | 0.2 |
| Water | 0.124 |
| Pine, with grain | 0.03 |
| Pine, across grain | 0.01 |
| Felt | 0.008 |
| Air | 0.005 |
[Pg 175]
Fig. 132.?Wall
construction of a refrigerator. 1, Porcelain enamel lining lock joint;
2, inside wood lining; 3, 3-ply red rope waterproof paper; 4, wool felt
deafening paper; 5, flaxlinum insulation; 6, dead air space; 7,
flaxlinum insulation; 8, wool felt deafening paper; 9, 3-ply red rope
waterproof paper; 10, outside wood case.Fig. 133.?Sectional view of a Thermos bottle.
Other illustrations of effective non-conductors will occur to every one; such as woolen clothing, wooden handles for hot objects, and the packing used in fireless cookers. A Thermos bottle is effective as a non-conductor of heat because the space between the double walls has the air exhausted from it (Figs. 133 and 134).
Of several objects in a cold room, some feel much colder to the touch than others, thus iron, marble, oil cloth, and earthenware will feel colder than woolen cloth, carpet, feathers, or paper. The first four objects feel cold because they are conductors, and conduct the heat away from the hand rapidly. The other substances named are non-conductors and hence remove heat from the hand less rapidly, and therefore do not feel so cold. In a similar way, if several hot objects are touched by the hand, the good conductors are the ones which will burn one most quickly by conducting heat rapidly to the hand. The non-conductors, however, will rarely burn one. Why are the handles of hot utensils often made of non-conducting materials such as wood, cloth, asbestos, etc.?
Fig. 134.?Cross-section of the vacuum flask in a Thermos bottle.
The radiation of heat is believed to be accomplished by means of waves in a medium called ether, which is invisible and yet pervades everything. Three of the most important characteristics of radiation are first, heat is transferred by radiation with the speed of light, or 186,000 miles per second. This fact is shown by the cutting off of both the sun's heat and light at the same instant during an eclipse of the sun. Second, radiant heat[I] travels in straight lines, while other modes of transferring heat may follow irregular paths. The straight line motion of radiant heat is shown by its being cut off where a screen is placed between the source of heat and the object sheltered. Third, radiant heat may pass through an object without heating it. This is shown by the coldness of the upper layers of the atmosphere and also by the fact that a pane of glass may not be heated appreciably by the heat and light from the sun which passes through it.
When radiant energy falls upon any object it may be (a) reflected at the surface of the object, (b) transmitted through the substance, (c), absorbed. All three of these effects occur in different degrees with different portions of the radiation. Well-polished surfaces are good reflectors. Rough and blackened surfaces are good absorbers. Transparent objects are those which transmit light well, but even they absorb some of the energy.
154. The Radiometer.?Radiant heat may be detected by means of the radiometer (Fig. 135). This consists of a glass bulb from which the air has been nearly exhausted.[Pg 178] Within it is a wheel with four vanes of mica or of aluminum mounted on a vertical axis. One side of each vane is covered with lampblack, the other being highly polished. when exposed to radiant heat from any source the vanes revolve with the bright side in advance.
The bulb is so nearly exhausted of air that a single molecule remaining may travel from the walls of the bulb to the vanes without coming in contact with another molecule.
The blackened sides absorb more heat than the highly polished sides. The air molecules striking these blackened sides receive more heat and so rebound with greater velocity than from the other side, thus exerting greater pressure. The blackened sides therefore are driven backward. If the air were not so rarified the air molecules would hit each other so frequently as to equalize the pressure and there would be no motion.
Fig. 135.?A radiometer.
Important Topics
1. Conduction in solids, liquids, gases.2. Non-conductors; uses, best non-conductors.
3. Radiation, three characteristics.
4. The sun's radiation, amount. The radiometer.
Exercises
1. Does clothing ever afford us heat in winter? How then does it keep us warm?2. Why are plants often covered with paper on a night when frost is expected?
3. Will frost form in the fall of the year sooner on a wooden or a cement sidewalk? Why? On which does ice remain longer? Why?
4. Why in freezing ice-cream do we put the ice in a wooden pail and the cream in a tin one?
5. Is iron better than brick or porcelain as a material for stoves? Explain.
6. Which is better, a good or a poor conductor for keeping a body warm? for keeping a body cool?
7. Should the bottom of a teakettle be polished? Explain.
8. How are safes made fireproof?
9. Explain the principle of the Thermos bottle.
10. Explain why the coiled wire handles of some objects as stove-lid lifters, oven doors, etc., do not get hot.
(5) Transmission of Heat in Fluids. Heating and Ventilation
155. Convection.?While fluids are poor conductors, they may transmit heat more effectively than solids by the mode called convection. To illustrate: if heat is[Pg 180] applied at the top of a test-tube of water, the hot water being lighter is found at the top, while at the bottom the water remains cold. On the other hand, if heat is applied at the bottom of the vessel, as soon as the water at the bottom is warmed (above 4?C.) it expands, becomes lighter and is pushed up to the top by the colder, denser water about it. This circulation of water continues as long as heat is applied below, until all of the water is brought to the boiling temperature. (See Fig. 136.)When a liquid or a gas is heated in the manner just described, the heat is said to be transferred by convection. Thus the air in the lower part of a room may receive heat by conduction from a stove or radiator. As it expands on being warmed, it is pushed up by the colder denser air about it, which takes its place, thus creating a circulation of the air in the room. (See Fig. 137.) The heated currents of air give up their heat to the objects in the room as the circulation continues. These air currents may be observed readily by using the smoke from burning "touch paper" (unglazed paper that has been dipped into a solution of potassium nitrate ["saltpeter"] and dried).
Fig. 136.?Convection in a liquid.
The chimney soon becomes filled with these heated gases. (See Fig. 138.) These are pushed upward by the pressure of the colder, denser air, because this colder air is pulled downward more strongly by the force of gravity than are the heated gases in the chimney.
Other things being equal, the taller the chimney, the greater the draft, because there is a greater difference between the weight of the gases inside and the weight of an equal volume of outside air.
Fig. 137.?Convection currents in a room.Fig. 138.?Fire place showing draft of a chimney.
Fig. 139.?Sea breeze.Fig. 140.?Land breeze.
158. The heating and ventilation of buildings and the problems connected therewith are matters of serious concern to all who live in winter in the temperate zone. Not only should the air in living rooms be comfortably heated, but it should be continually changed especially in the crowded rooms of public buildings, as those of schools, churches, and assembly halls, so that each person may be supplied with 30 or more cubic feet of fresh air per minute. In the colonial days, the open fire place afforded the ordinary[Pg 183] means for heating rooms. This heated the room mainly by radiation. It was wasteful as most of the heat passed up the chimney. This mode of heating secured ample ventilation. Fire places are sometimes built in modern homes as an aid to ventilation.
Benjamin Franklin seeing the waste of heat in the open fire places devised an iron box to contain the fire. This was placed in the room and provided heat by conduction, convection, and radiation. It was called Franklin's stove and in many forms is still commonly used. It saves a large part of the heat produced by burning the fuel and some ventilation is provided by its draft.
Fig. 141.?Heating and ventilating by means of a hot-air furnace.
Fig. 142.?A hot-water system of heating.
[Pg 186]
160. Hot-water Heating.?In hot-water heating a furnace arranged for heating water is placed in the basement. (See Fig. 142.) Attached to the top of the heater are pipes leading to the radiators in the various rooms; other pipes connect the radiators to the bottom of the boiler. The heater, pipes, and radiators are all filled with water before the fire is started. When the water is warmed, it expands and is pushed up through the pipes by the colder water in the return pipe. The circulation continuing brings hot water to the radiator while the cooled water returns to the heater, the hot radiators heating the several rooms.
161. Steam Heating.?In steam heating a steam boiler is connected to radiators by pipes. (See Fig. 143.) The steam drives the air out of the pipes and radiators and serves as an efficient source of heat. Heating by steam is quicker than heating with hot water. It is therefore preferred where quick, efficient heating is required. Hot water is less intense and more economical in mild weather and is often used in private homes.
Fig. 144.?Heating by an indirect radiator with side-wall register.
Fig. 145.?An automatic air valve.
Fig. 146.?An automatic vacuum valve.
Fig. 147.?Plenum hot-blast system with temperature regulation.
Fig. 148.?A thermostat. (Johnson System.)
165. The Thermostat.?One of the many examples of the expansion of metals is shown in one form of the thermostat (Fig. 148) in which two pieces of different metals[Pg 190] and of unequal rates of expansion, as brass and iron, are securely fastened together.
The thermostatic strip T moving inward and outward, as affected by the room temperature, varies the amount of air which can escape through the small port C. When the port C is completely closed (Fig. 148a) the full air pressure collects on the diaphragm B which forces down the main valve, letting the compressed air from the main pass through the chamber D into chamber E as the valve is forced off its seat. The air from chamber E then passes into the branch to operate the damper.
When port C is fully open (Fig. 148b) the air pressure on diaphragm B is relieved, the back pressure in E lifts up the diaphragm and the air from the branch escapes out through the hollow stem of the main valve, operating the damper in the opposite direction from that when C is closed.
Important Topics
1. Transmission of heat in fluids.2. Convection. Drafts of a chimney. Land and sea breezes.
3. Heating and ventilation of buildings.
(a) By hot air.
(b) Hot-water heating.
(c) Steam heating.
(d) Direct and indirect heating.
(e) Vacuum steam heating.
(f) The plenum system.
(g) The thermostat.
Exercises
1. Is a room heated mainly by conduction, convection, or radiation, from (a) a stove, (b) a hot-air furnace, (c) a steam radiator?2. Name three natural convection currents.
3. Explain the draft of a chimney. What is it? Why does it occur?
[Pg 191]
4. Make a cross-section sketch of your living room and indicate the convection currents by which the room is heated. Explain the heating of the room.
5. Make a sketch showing how the water in the hot-water tank in the kitchen or laundry is heated. Explain your sketch, indicating convection currents.
6. Is it economical to keep stoves and radiators highly polished? Explain.
7. If you open the door between a warm and a cool room what will be the direction of the air currents at the top and at the bottom of the door? Explain.
8. If a hot-water heating system contains 100 cu. ft. of water how much heat will be required to raise its temperature 150?F.?
9. Why does a tall chimney give a better draft than a short one?
10. Explain how your school room is heated and ventilated.
11. Should a steam or hot-water radiator be placed near the floor or near the ceiling of a room? Why?
12. In a hot-water heating system an open tank connected with the pipes is placed in the attic or above the highest radiator. Explain its use.
(6) The Moisture in the Atmosphere, Hygrometry
166. Water Vapor in the Air.?The amount of water vapor present in the air has a marked effect upon the weather and the climate of a locality. The study of the moisture conditions of the atmosphere, or hygrometry, is therefore a matter of general interest and importance. The water vapor in the atmosphere is entirely due to evaporation from bodies of water, or snow, or ice. In the discussion of evaporation, it is described as due to the gradual escape of molecules into the air from the surface of a liquid. This description fits exactly the conditions found by all careful observers. Since the air molecules are continually striking the surface of the liquid, many of them penetrate it and become absorbed. In the same manner many vapor molecules reenter the liquid, and if[Pg 192] enough vapor molecules are present in the air so that as many vapor molecules reenter the liquid each second as leave it, the space above the liquid is said to be saturated as previously described. (See Art. 18.)167. Conditions for Saturation.?If a liquid is evaporating into a vacuum, the molecules on leaving find no opposition until they reach the limits of the vessel containing the vacuum. Evaporation under these conditions goes on with great rapidity and the space becomes saturated almost instantly. If, however, air be present at ordinary pressure, many of the ordinary water vapor molecules on leaving are struck and returned to the water by the air molecules directly above. Those escaping gradually work their way upward through the air. This explains why it is that our atmosphere is not often saturated even near large bodies of water, the retarding effect of the air upon the evaporation preventing more than the layers of air near the water surface becoming saturated.
Just as the amount of salt that can be held in solution in a liquid is lessened by cooling the solution (Art. 26), so the amount of water vapor that can be held in the air is lessened by lowering its temperature. If air not moist enough to be saturated with water vapor is cooled, it will, as the cooling continues, finally reach a temperature at which it will be saturated or will contain all the water vapor it can hold at this temperature. If the air be still further cooled some of the water vapor will condense and may form fog, dew, rain, snow, etc., the form it takes depending upon where and how the cooling takes place.
168. The Formation of Dew.?If the cooling of the atmosphere is at the surface of some cold object which lowers the temperature of the air below its saturation point, some of its moisture condenses and collects upon the[Pg 193] cold surface as dew. This may be noticed upon the surface of a pitcher of ice-water in summer. At night, the temperature of grass and other objects near or on the ground may fall much faster than that of the atmosphere owing to the radiation of heat from these objects. If the temperature falls below the saturation point, dew will be formed. This natural radiation is hindered when it is cloudy, therefore little dew forms on cloudy nights. Clear nights help radiation, therefore we have the most dew on nights when the sky is clear. If the temperature is below freezing, frost forms instead of dew.
169. Formation of Fog.?If the cooling at night is great enough to cool the body of air near the earth below the saturation temperature, then not only may dew be formed, but some moisture is condensed in the air itself, usually upon fine dust particles suspended in it. This constitutes a fog. If the cooling of the body of air takes place above the earth's surface as when a warm moist current of air enters a colder region, e.g., moves over the top of a cold mountain, or into the upper air, then as this air is cooled below its saturation point, condensation upon fine suspended dust particles takes place, and a cloud is formed. If much moisture is present in the cloud, the drops of water grow in size until they begin to fall and rain results; or if it is cold enough, instead of rain, snowflakes will be formed and fall. Sometimes whirling winds in severe thunderstorms carry the raindrops into colder and then warmer regions, alternately freezing and moistening the drops or bits of ice. It is in this way that hail is said to be formed.
170. The Dew Point.?The temperature to which air must be cooled to saturate it or the temperature at which condensation begins is called the dew point. This is often determined in the laboratory by partly filling a polished[Pg 194] metal vessel with water and cooling the water by adding ice until a thin film of moisture is formed upon the outer surface. The temperature of the surface when the moisture first forms is the dew point.
171. The Humidity of the Atmosphere.?After the dew point has been obtained, one may compute the relative humidity or degree of saturation of the atmosphere, from the table given below. This is defined as the ratio of the amount of water vapor present in the air to the amount that would be present if the air were saturated at the same temperature.
For example, if the dew point is 5?C. and the temperature of the
air is 22?C., we find the densities of the water vapor at the two
temperatures, and find their ratio: 6.8/19.3 = 35 per cent. nearly.
Determinations of humidity may give indication of rain or frost and are
regularly made at weather bureau stations. They are also made in
buildings such as greenhouses, hospitals, and schoolhouses to see if the
air is moist enough. For the most healthful conditions the relative
humidity should be from 40 per cent. to 50 per cent.
Weight of Water (w) in Grams Contained in 1 Cubic Meter of Saturated Air at Various Temperatures (t?)C.| t?C. | w |
| -10 | 2.1 |
| - 9 | 2.4 |
| - 8 | 2.7 |
| - 7 | 3.0 |
| - 6 | 3.2 |
| - 5 | 3.5 |
| - 4 | 3.8 |
| - 3 | 4.1 |
| - 2 | 4.4 |
| - 1 | 4.6 |
| 0 | 4.9 |
| 1 | 5.2 |
| 2 | 5.6 |
| 3 | 6.0 |
| 4 | 6.4 |
| 5 | 6.8 |
| 6 | 7.3 |
| 7 | 7.7 |
| 8 | 8.1 |
| 9 | 8.8 |
| 10 | 9.4 |
| 11 | 10.0 |
| 12 | 10.6 |
| 13 | 11.3 |
| 14 | 12.0 |
| 15 | 12.8 |
| 16 | 13.6 |
| 17 | 14.5 |
| 18 | 15.1 |
| 19 | 16.2 |
| 20 | 17.2 |
| 21 | 18.2 |
| 22 | 19.3 |
| 23 | 20.4 |
| 24 | 21.5 |
| 25 | 22.9 |
| 26 | 24.2 |
| 27 | 25.6 |
| 28 | 27.0 |
| 29 | 28.6 |
| 30 | 30.1 |
Fig. 149.?Wet and dry bulb hygrometer.Fig. 150.?A dial hygrometer.
A form of hygrometer in common use is shown in Fig. 150. In this
device, a thin strip of hygroscopic material (as a piece of goose quill)
is formed into a spiral coil. One end of this is fastened to a post.
The other end carried a hand or pointer. The latter moves[Pg 196]
over a printed scale and indicates directly the relative humidity. Its
indications should be tested by comparing its readings with the results
of dew-point determinations. The position of the pointer may be adjusted
by turning the post.
Important Topics
1. Water vapor in the air. Cause and effect.2. Formation of dew, fog, rain, and snow.
3. Dew point, relative humidity.
4. Use of the dry- and wet-bulb hygrometer. Goose-quill hygrometer.
Exercises
1. How is the relative humidity of the air affected by warming it? Explain.2. How does the white cloud of steam seen about a locomotive in cold weather differ from fog? Explain.
3. In cold weather is the relative humidity of air out of doors and indoors the same? Explain.
4. Compare the relative humidity of air in a desert and near the ocean.
5. Look up the derivation of the term "hygrometer." Give the use of the instrument.
6. Find the relative humidity of air at 20?C. if its dew point is at 10?C.
7. How may the relative humidity of the air in a home be increased?
8. What is the effect of high humidity in the summer upon human beings? How do you explain this?
9. Does dew fall? Explain how dew is formed?
10. In what respects is a cloud similar to a fog? In what respects different?
11. Why are icebergs frequently enveloped in fog?
12. Does dew form in the day time? Explain.
(7) Evaporation
173. Effects of Evaporation.?In Art. 19 the cooling effect of evaporation is mentioned and some explanation is made of the cooling effect observed. Since evaporation is employed in so many ways, and since its action is simply explained by the study we have made of molecular motions[Pg 197] and molecular forces, it may be well to consider this subject further.
Take three shallow dishes, and place in one a little water, in
another some alcohol, and some ether in the third, the liquids being
taken from bottles that have stood several hours in the room so that all
are at the same temperature. After a short time take the temperature of
the three liquids. Each will be at a lower temperature than at first,
but of the three the ether will be found to be the coolest, alcohol
next, and the water nearest its first temperature. It will be noticed
also that the ether has evaporated most in the same time. Similar
effects may be observed by placing a few drops of each of these three
liquids upon the back of one's hand, or by placing a few drops in turn
upon the bulb of a simple air thermometer.
174. Cooling Effect of Evaporation.?The molecules that leave
an evaporating liquid are naturally the swiftest moving ones, that is,
the ones having the highest temperature, so their escape leaves the
liquids cooler than before, and the one whose molecules leave fastest is
naturally the one that becomes coldest, that is, the ether, in the
experiment of Art. 173. If no air pressure were exerted upon the surface
of the liquid, the escape of the molecules would be much increased and
the temperature of the liquid would be lowered rapidly.
To test this, fill a thin watch glass with ether and place it over
a thin slip of glass with a drop of cold water between the two. Now
place this apparatus under the receiver of an air pump and exhaust the
air. The rapid evaporation of the ether so lowers its temperature, that
often the drop of water is frozen. The lowest temperatures are obtained
by evaporating liquids at reduced pressure.
Onnes by evaporating liquid helium at a pressure of about 1.2 mm.
reached the lowest temperature yet attained, -456?F., or -271.3?C.
If four thermometers are taken, the bulbs of three being wetted
respectively with ether, alcohol, and water the fourth being dry, on
vigorously fanning these, the moistened thermometers show that they have
been cooled while the dry one is unaffected.
[Pg 198]This indicates that fanning a dry body at the temperature of the air does not change its temperature. Fanning does increase evaporation by removing the air containing the evaporated molecules near the surface of the liquid so that unsaturated air is continually over the liquid. If a pint of water is placed in a bottle and another pint in a wide pan the latter will become dry much sooner because of the greater surface over which evaporation can take place. Application of this is made at salt works where the brine is spread out in shallow pans.
175. Rate of Evaporation.?The rate of evaporation is affected by several factors. These have been illustrated in the preceding paragraphs. To briefly summarize:
The rate of evaporation of a liquid is affected by?
(a) The nature of the liquid.
(b) The temperature of the liquid.
(c) The pressure upon its evaporating surface.
(d) The degree of saturation of the space into which the liquid is evaporating.
(e) The rate of circulation of air over its surface.
(f) The extent of surface exposed to evaporation.
176. Molecular Motion in Solids.?Evidence of molecular motion in liquids is given by expansion on heating, evaporation, and diffusion. Do any of these lines of evidence apply to solids? It is a fact of common experience that solids do become larger on heating. Spaces are left between the ends of rails on railroads so that when they expand in summer they will not distort the track. Iron tires are placed on wheels by heating them until they slip on easily. Then on cooling, the iron shrinks and presses the wheel tightly. Many common demonstrations of expansion are found in lecture rooms. The fact of the evaporation of a solid is often detected by noticing the odor of a substance. The odor of moth balls is[Pg 199] one example. Camphor also evaporates. Heated tin has a characteristic odor noted by many. Ice and snow disappear in winter even though the temperature is below freezing. Wet clothes, "freeze dry," that is, dry after freezing, by evaporation. A few crystals of iodine placed in a test-tube and gently heated form a vapor easily seen, even though none of the iodine melts. Where the vapor strikes the side of the tube, it condenses back to dark gray crystals of iodine. This change from solid directly to gas and back again without becoming liquid is called sublimation. A number of solids are purified by this process.
Important Topics
1. Cooling effect of evaporation, rate of evaporation affected by six conditions.2. Effects of molecular motion in solids: (a) Expansion, (b) Evaporation, (c) Sublimation.
Exercises
1. Does sprinkling the streets or sidewalks cool the air? Why?2. Give an illustration for each of the factors affecting evaporation.
3. Give an illustration for each of the three evidences of molecular motions in solids.
4. Since three-quarters of the earth's surface is covered with water, why is not the air constantly saturated?
5. If the air has the temperature of the body, will fanning the perfectly dry face cool one? Explain. Will the effect be the same if the face is moist? Explain.
6. What is the cause of "Cloud Capped" mountains?
7. Why does the exhaust steam from an engine appear to have so much greater volume on a cold day in winter than on a warm one in summer?
8. What causes an unfrozen pond or lake to "steam" on a very cold day in winter, or on a very cool morning in summer?
9. As the air on a mountain top settles down the sides to places of greater pressure, how will its temperature be affected? its relative humidity? Explain.
10. On our Pacific coast, moist winds blow from the west over the mountains. Where will it rain? Where be dry? Explain.
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