HEAT, ITS PRODUCTION AND TRANSMISSION
(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 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.
[Illustration: 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.
[Illustration: 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_.
=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.
[Illustration: 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?
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 _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.
[Illustration: 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 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.
(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.
[Illustration: 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 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 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 _V_{1}/V_{2} = T_{1}/T_{2}_. 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.
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.?
[Illustration: 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?
(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 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.
[Illustration: 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:
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.
[Illustration: 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.
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 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.)
[Illustration: 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.)
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.)
[Illustration: 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
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.
=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_.
[Illustration: 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 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.
[Illustration: FIG. 131.--Water is a poor conductor of heat.]
These figures are averages taken mainly from the Smithsonian Physical
Tables:
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
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.
[Illustration: 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.]
[Illustration: 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 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.?
[Illustration: 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 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.
[I] Radiant heat is really _radiant energy_ and becomes heat when
it is absorbed by a body.
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. 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.
[Illustration: 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 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.
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 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).
[Illustration: 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 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.
[Illustration: FIG. 137.--Convection currents in a room.]
[Illustration: 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 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.)
[Illustration: FIG. 139.--Sea breeze.]
[Illustration: 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 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.
[Illustration: 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 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_.
[Illustration: FIG. 142.--A hot-water system of heating.]
[Illustration: FIG. 143.--One-pipe system of steam heating.]
=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.
[Illustration: 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 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.
[Illustration: FIG. 145.--An automatic air valve.]
[Illustration: 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 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.
[Illustration: 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 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.
[Illustration: 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 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. 148_a_) 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. 148_b_) 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?
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 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 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 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
--------+------
=172. Wet and Dry Bulb Hygrometer.=--A device for indicating the
relative humidity of the air is called an _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.
[Illustration: FIG. 149.--Wet and dry bulb hygrometer.]
[Illustration: 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
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 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.
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 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.