HEAT.
270. =Heat and Cold.=--In common language we speak of heat and
cold as two distinct and opposite things. That this is not
strictly correct may be shown by the following experiment: Take
three vessels, and fill the first with ice-cold water, the second
with hot water, and the third with tepid water. If you place
your right hand in the first and the left in the second, and let
them remain a little time, on taking them out and plunging them
together into the third vessel, the water in it will feel warm to
the right hand and cold to the left. So the air of a cellar seems
warm to you in winter and cold in summer in contrast with the air
outside. For the same reason water of a temperature that would
ordinarily be refreshingly cool to us seems warm when drank after
eating ice-cream. It is manifest, then, that there is no fixed
dividing-line between heat and cold. There is, in fact, no such
thing as cold. Substances are cold from being deprived of heat; and
no substance ever has all its heat taken from it. Sir Humphrey Davy
proved that there is heat in ice by rubbing two pieces together in
a very cold room. They were gradually melted. Now this was not done
by the air, for that was at a temperature below the freezing point.
The heat which melted the ice came from the ice itself by means of
the rubbing.
271. =Nature of Heat.=--There are two theories in regard to the
nature of heat. One is that heat is an imponderable (§ 16), and of
course a very subtile substance, which pervades all matter. Its
particles are supposed to repel each other strongly, and hence
they have a tendency to diffuse themselves, and to separate the
particles of matter from each other. It is in this way that they
are supposed to occasion the expansion of substances. The other
supposition, which is most commonly received, is that heat is a
vibration of the particles of bodies, and that it passes from these
to bodies less warm through a subtile fluid called ether, supposed
to fill all space. You see that if this be the true theory, there
is some analogy between heat and sound.
[Illustration: Fig. 191.]
272. =Sources of Heat.=--The principal of the sources of heat on
our earth is the _sun_, though that body is ninety-five millions
of miles distant from us. As the heat, in traveling all this long
journey, is becoming more and more diffused; or, in other words, as
its rays are all the way separating from each other more and more,
we can have no conception of the concentrated heat that exists
in the sun itself. We can, however, approximate to the idea by
observing the effects of heat when some of its separated rays are
gathered to a point by a powerful lens, as represented in Fig.
191. A lens which concentrated the heat ten thousand times melted
platinum, gold, quartz, etc., in a few seconds. And as the heat
at the sun is supposed to be thirty times more concentrated than
this, none of the most solid substances of our earth would remain
solid if they were there, but would be some of them liquid, and
others even in a state of vapor. The heat which the sun constantly
radiates to the earth pervades all substances, producing motion,
and awakening life every where, so that, in the expressive language
of the Bible, "There is nothing hid from the heat thereof."
Another source of heat is _within the earth itself_. It has been
found as we go down into the earth there is a constant increase of
temperature the farther we go. This internal heat is attributed in
part to subterranean fires and various chemical actions. We see
here and there external evidences of the operation of these causes
in the eruptions of volcanoes, the boiling springs, the jets of
steam and sulphureous vapors, etc. But that the heat in our earth
which comes from these subterranean sources is small compared with
that which comes from the sun, is seen in the fact that the rate
of increase of heat at great depths is much less than it is nearer
the surface. This would seem to show that although fires within
the earth may have considerable influence in heating its crust, on
which we live, it derives the most of its heat from the sun, at
least to a very great depth.
How great a source of heat _electricity_ is we know not, but that
considerable heat comes from this source is evident from the
melting and burning effect which we often see resulting from the
passage of the electric fluid.
Another very common source of heat is _chemical action_. We see it
continually produced in chemical experiments. Combustion, which, as
will be shown to you in the Second Part of this Series, is nothing
but an example of chemical action, is the most common of all the
chemical sources of heat. Animal heat is also, for the most part, a
result of chemical action.
_Mechanical action_ is a common source of heat. The rubbing of
a match producing heat enough to occasion flame is a familiar
example. The spark produced in what is called striking fire is the
burning of a particle of steel set on fire by the blow. The Indian
was accustomed to light his fire by the rubbing together of two dry
sticks till he learned an easier way from civilized neighbors; and
the blacksmith, previous to the invention of phosphorus matches,
often lighted his fire by touching a sulphur match to a nail made
red-hot by rapid and continued hammering. Machinery has sometimes
been set on fire by friction, and the water around a mass of metal
has been so heated by boring as even to boil. If you stretch a
piece of India rubber several times in quick succession, and then
apply it to your lips, you will perceive that the motion has warmed
it.
273. =Relations of Heat and Light.=--Heat is sometimes alone, and
is sometimes in intimate union with light. All substances have
some amount of heat, and it passes from them to other bodies in
their neighborhood that happen to have less heat in them. In doing
this it may or may not have the company of light. In the radiation
of heat from a stove, unless it be heated to redness, there is
no light with the heat; but from an open, burning fire the light
and heat come together. But the rays of the sun give us the best
example of the union of light and heat. Traveling together at an
equal pace they are most curiously mingled, as you will see when I
come to speak particularly of light.
I will now proceed to notice the principal effects of heat; viz.,
expansion, liquefaction, and vaporization.
[Illustration: Fig. 192.]
274. =Expansion in Solids.=--Heat, you have seen in § 23, acts in
opposition to the attraction of cohesion, tending to separate the
particles, and so produces an expansion of any substance. This may
be exemplified in the experiment represented in Fig. 192, in which
A B is an iron rod, which is of such a size that at the ordinary
temperature it will fit into the space, C D, in a bar of iron, and
easily pass through the hole, E. If the rod be heated it will be
enlarged or expanded in all directions, so that it will neither
fit into C D nor pass into the hole, E. When the wheel-wright
puts a tire upon a wheel he uses the expansion of heat to make it
fit tightly and firmly. The tire is made a little too small to
have it fit upon the wheel as it is. But by being heated it is
so expanded that it will readily go on to the wheel, and then in
contracting as it cools it so compresses the fellies as to hold on
very tightly. Water is poured on to cool the iron quickly, and thus
prevent it from burning the wood. Iron hoops are put on barrels
in a similar manner, the compression caused by their contraction
binding the staves together very strongly. So in fastening the
plates of boilers together, the rivets are put in red-hot, so that
in their contraction they may press the plates closely together.
If an iron gate just shuts into its place in cold weather, its
expansion will prevent its shutting when warm weather comes. In
order to avoid this difficulty, calculation must be made in fitting
it for its place for the expansion to which it will be subjected
by heat. So in laying the rails of a railroad in cold weather
care must be taken not to put the ends too near together. In
constructing iron bridges the expansion by heat must be calculated
for in the arrangement. Nails often become loose after the lapse of
years from the wear of the wood around them, occasioned by their
alternate expansion and contraction. The leaking of gas-pipes in
the earth is often undoubtedly caused by the loosening of the
joints from contraction and expansion of the pipes by varying
temperatures of the soil, especially where they are not laid very
deep. If a stopper stick fast in a bottle it can be loosened by
the application around the neck of a cloth dipped in hot water,
because the neck becomes expanded at once by the heat. A similar
expedient was once very ingeniously made use of in repairing the
machinery of the steamer _Persia_ at sea, and was perhaps the
means of saving the vessel and the lives of all on board. The
accident which occurred was the breaking of the port crank-pin of
the engine. The problem to be solved was the removal of this pin,
which weighed nearly a ton, and the substitution of a sound one
which they had on hand in its place. But it was found impossible to
start the broken pin from its socket with all the force which could
be brought to bear upon it by a sort of battering-ram constructed
extemporaneously for the purpose. It was determined now to try
the expansive force of heat. An iron platform was built under the
socket, and a brisk fire made upon it. The socket soon expanded,
and the pin was now readily knocked out by the battering-ram, just
as the stopper of the bottle is easily removed when the neck is
heated. The walls of a very large building in Paris, which had
bulged out and were in danger of falling, were restored to their
upright position by the expansion of heat.
[Illustration: Fig. 193.]
It was done in this way: Long rods of iron were run through the
walls after the plan represented in Fig. 193 (p. 213), their ends
being made with a screw-thread, with nuts fitted to them. The rods
marked _a_ were first heated, and as they lengthened the nuts were
screwed up tight to the walls. On cooling, their contraction would
of course draw the walls together. The other bars, _b_, were now
heated and managed in the same way. The one set, you see, were made
to hold on by their nuts to what had already been gained, while the
other were expanding. By many repetitions of this process the walls
were righted and the building saved. The same mode has been adopted
successfully in other cases of a similar character.
275. =Expansion in Liquids.=--Liquids are expanded by heat more
than solids are. But they are very unequally expanded by it. Thus
water is expanded more than twice as much as mercury, and alcohol
six times as much. We have a frequent example of the expansion of
water by heat in our kitchens. If the tea-kettle be put over the
fire filled to the brim, it will run over long before the water
begins to boil. All liquids occupy more space in summer than in
winter, and in the former case weigh less--that is, have less of
real substance in them than in the latter. If, therefore, alcohol,
or oil, or molasses be bought by the gallon in winter and sold in
summer, there will be a profit afforded by the expansion. Twenty
gallons of alcohol in winter becomes twenty-one in mid-summer.
[Illustration: Fig. 194.]
The influence of the expansion of heat upon the specific gravity
of liquids may be very prettily shown by the following experiment:
Let some little bits of amber--a substance which is nearly of the
same specific gravity with water--be thrown into water in a glass
vessel, and let the water be heated, as represented in Fig. 194, by
a spirit-lamp. That portion of the water which is heated passes
upward because it is made specifically lighter, and colder water
continually comes down to take its place. The upward and downward
currents are as indicated by the arrows, the upward passing up in
the middle, the downward coming down at the sides. This will be
made manifest by the little bits of amber.
276. =Thermometers.=--It is the expansion of liquids by heat that
in the thermometer gives us the measure of temperature. The liquid
metal mercury is commonly used for this purpose, and answers well
except in the extreme cold of the arctic regions. There, as mercury
becomes solid at 39 degrees below zero, it is necessary to use a
thermometer with alcohol in it, as this fluid can not be frozen
by any degree of cold. The operation of the thermometer is simply
this: Heat expands the fluid in the bulb, and the only way in which
it can occupy more space as it expands is by rising in the tube.
The abstraction of heat, on the other hand, causes contraction, and
of course a proportionate falling of the fluid.
277. =Fahrenheit's Thermometer.=--The thermometer was invented in
the beginning of the seventeenth century, but it is not decided
who was the inventor. There may have been in this case, as in
others, more inventors than one, the same ideas having, perhaps,
entered several inquiring minds at the same time. Various fluids
were used by different persons. Sir Isaac Newton used linseed
oil. Fahrenheit, a native of Hamburg, who flourished in the first
part of the last century, was the first to use mercury. Though
various propositions were made by Newton and others in regard to
the measurement of heat by thermometers, no thermometric scale
seems to have met with general reception till that of Fahrenheit's,
which was put forth about 1720. The plan of it is this: His zero
is the point at which the mercury stood in the coldest freezing
mixture that he could make; and he supposed that this was the
greatest possible degree of cold, as it was the greatest that he
knew. He next found the point at which the mercury stood in melting
ice. This he called the freezing point, because the temperature
is the same in water passing into the solid from the fluid state
as in water passing into the fluid state from the solid. In other
words, this point in the scale marks the transition line between
the two states. From this point Fahrenheit marked off 32 equal
spaces or degrees down to zero. He now found the point at which
the mercury stands in boiling water, and called this the boiling
point. Marking off the space on the scale between this, and the
freezing point in the same manner, there are 180 degrees--that is,
the boiling point is 212 degrees above zero. The degrees above zero
are commonly designated by the mark +, plus; and those below by the
mark -, minus. Thus, +32° signifies 32 degrees above zero, and -32°
signifies 32 degrees below.
[Illustration: Fig. 195.]
278. =Other Thermometers.=--Fahrenheit's thermometer is the
one commonly used in this country. But there are several other
thermometers on different scales, as the Centigrade, Reaumur's, and
De Lisle's. In Fig. 195 you see the plans of the scales of these
thermometers placed side by side. In the Centigrade thermometer,
which is in use in France, and indeed in a large part of Europe,
the zero, you see, is placed at the freezing point; and the space
between this and the boiling point is divided into 100 degrees,
which gives it the name Centigrade. Reaumur's, which is in use
in Russia, has the same zero, but he has only 80 degrees from
this to the boiling point. De Lisle's, which has gone entirely
out of use, has its zero at the boiling point. The arrangement of
Fahrenheit, although its zero is a mere arbitrary point, is, on the
whole, the best, because its degrees are of such a size that they
mark differences of temperature with sufficient minuteness for all
practical purposes of an ordinary character without resorting to
fractional parts.
[Illustration: Fig. 196.]
279. =Expansion in Aeriform Substances.=--Heat produces a vastly
greater expansive effect in air, the gases, and vapors, than it
does in liquids. The expansion of air by heat may be shown very
prettily in this way: Take a glass tube that has a bulb on one
end, and, placing the other open end in water (as represented in
Fig. 196), apply the palm of your hand to the bulb. The heat of
the hand being communicated to the bulb will expand the air, and
so, as you see, bubbles of air will escape through the water. On
removing the hand, and allowing the bulb to cool, the air in it
will be condensed, and water will pass up in the tube in proportion
to the amount of air which has escaped. A bladder partly filled
with air will be made to swell out to plumpness if it be heated
sufficiently, and a full one may be so heated as to burst from the
expansion of the air. Porous wood, as chestnut, snaps very much
when burned, because the heat expands the air contained in the
pores.
280. =Balloons.=--The first balloons that were used were filled
with heated air. You have already seen, in § 149, why it is that
balloons rise. Now in the hot-air balloon it is the expansion of
the air by heat that makes it lighter than the surrounding air. Of
course such a balloon is not as effective as the gas balloon, for
the air within it loses its comparative lightness as it becomes
cooled; while the gas which is used, being very much lighter than
air at the same temperature, does not lose its lightness as the
balloon goes up. You learned in § 152 that the atmosphere becomes
thinner as we go upward. The gas balloon, therefore, rises until it
arrives at that point where the air is of about the same specific
gravity with the gas, and there it stops. It is made to descend
by letting out some of the gas from a valve. Gas was not used for
balloons till 1782. Hydrogen gas was employed at first, being over
fourteen times lighter than air. Of late the common burning gas,
carbureted hydrogen, has been generally used, because it can be so
readily obtained where there are gas-works.
[Illustration: Fig. 197.]
281. =Currents in the Air from Heat.=--Heat is the grand mover
of the atmosphere. Any portion of it that becomes warmer than
surrounding portions rises, or rather is pushed up, for the same
reason that a hot-air balloon rises, the only difference between
the two cases being that in the one the air is confined, and in the
other is left free, and so becomes diffused. And it is this rising
of the air from expansion that causes nearly all the movements
that we witness in the air. We see this exemplified in various
ways wherever there is a fire. The air that is heated by the fire
is forced upward by the colder air, which, on the principles of
specific gravity, seeks to get below the warmer and lighter air.
The hot air that comes through the registers of a furnace is pushed
up by colder air below. For the same reason the heated air around a
stove-pipe is constantly going upward. This is very prettily shown
by the toy represented in Fig. 197 (p. 218), which is a paper cut
spirally, and suspended, as you see, upon the point of a wire. The
upward current makes the paper revolve rapidly around the wire. It
is from the rising of warm air that the galleries of a church are
warmer than the space below. In a common room the disposition of
the air is continually to have its warmest portions above and the
colder below. It is for this reason that we have our arrangements
for producing or introducing heat at as low a point as possible.
282. =Chimneys.=--We speak of the _draught_ of a chimney, and we
say of one that does not smoke that it _draws_ well, as if the
smoke were in some way actually drawn up. But the same principles
apply here as are developed in § 281. The smoke, which is a
combination of heated air and gases with some solid matters in a
fine state, is _forced_ up the chimney. When a chimney does not
draw well we open a door or a window for a little while until
the fire gets thoroughly agoing. Why is this? It is that we may
have denser air than there is in the room, so that the smoke may
be pushed up more forcibly. When the chimney becomes well heated
there is ordinarily no difficulty, because then the smoke in it
is not obliged to part with much of its heat to the walls of the
chimney, and therefore is so much lighter than the air in the room
that it is very easily forced upward. The principal reason that a
stove-pipe generally draws better than a chimney is that there is
much less heat expended in establishing and maintaining the upward
current. Especially is this true if the chimney be a large one. In
such a case there are both a great extent of brick and a large body
of air to be heated to establish the upward current, and these must
be kept warm in order to maintain it.[3]
[Illustration: Fig. 198.]
[Illustration: Fig. 199.]
283. =Winds.=--If you open a door of a heated room a candle held
near the floor will have its flame blown inward, while one held
near the top of the door will have its flame blown toward the cold
entry. Here you have a good illustration of the manner in which
winds are produced. Wherever the wind blows it is air pushing
out of the way other air that is warmer, in order that it may,
in obedience to gravitation, get as near the earth as possible.
Take, for example, the land and sea breezes, as they are called.
During a hot summer's day the sun heats the earth powerfully,
while the ocean receives but little of its heat. The heated land
heats the air above it; and as the air over the ocean is cooler,
and therefore heavier, it pushes upward the air of the land, for
the same reason that water pushes up oil; and as this goes on
continuously a regular current is established. The wind blows in
upon the land, as represented in Fig. 198, while the warmer air
passes upward into the higher regions of the atmosphere, and turns
toward the sea. The arrows show the course of the currents. The
resemblance of all this to the effect upon the candle held near
the open door is very obvious, the cold air from the entry blowing
in below representing the breeze from the ocean, and the warm air
of the room blowing out above representing the passage of the warm
air of the land out toward the ocean. At night this is apt to be
reversed. The earth becomes cooled, and with it the air that is
over it. The result is that the cooled air of the land now pushes
upward the warmer air of the sea, as seen in Fig. 199.
[Illustration: Fig. 200.]
[Illustration: Fig. 201.]
284. =Winds as Affected by the Rotation of the Earth.=--The heat
of the vertical sun upon the tropics causes a rise of heated air
into the upper regions, while there is a rush of colder air toward
the equator from both north and south. This effect is represented
in Fig. 200 (p. 221), E being the sun, N the north pole, and S
the south pole. An effect similar to that represented in Figs.
198 and 199 is produced here, but it is on a much larger scale.
But the diagram does not present the matter in its true light in
all respects. The prevailing winds in the equatorial regions are
not north and south winds, as would appear from this diagram; but
they are from the northeast and southeast. I will explain this by
Fig. 201. As the earth turns on its axis it is plain that there
is no part of the surface of the earth that moves so rapidly as
the equator, E W, for that moves in a larger circle than any other
part. And the nearer you go to either pole, N or S, the less is
the rapidity of the revolution. Now the atmosphere, as stated in §
188, partakes of the motion of the earth. The air, therefore, at
the equator is moving from west to east with the rotation of the
earth faster than it is any where else, and the nearer you go to
either pole the slower is its motion. It follows from this that
any portion of air blowing from the north or the south toward the
equator, as it comes from where it was moving east slower than air
at the equator is, would from its lesser momentum lag behind the
air of the equator, the wind would be curved toward the westward,
as indicated by the arrows. The result would be that the northern
wind would be converted into a northeaster, and the southern into
a southeaster. All this can be made more clear with a globe, or,
indeed, with any round object.
285. =Liquefaction.=--The change of solids into liquids is one of
the most observable effects of heat. This change requires different
degrees of heat in different substances. Thus while iron melts at
the high heat of 2786°, lead melts at 633°, sulphur at 239°, ice
at 32°, and mercury at 39° below zero. Mercury is never found in a
solid state, but it sometimes becomes solid in the arctic regions
when carried there and exposed in the open air. We are apt to think
of water as being in a more natural state when liquid than when it
is solid, just as we think of iron as being naturally solid and
mercury as naturally liquid. But in all these cases the state of
the substance depends on its temperature, and this is varied by
circumstances. Water at the equator is always liquid, and the idea
of ice there is exceedingly unnatural; while near the poles it is
the reverse, ice and snow reigning every where throughout the whole
year.
286. =Evaporation.=--There are two ways in which the change of
a liquid into a vapor occurs. One is a rapid change when heat
is so applied as to raise the liquid to its boiling point. This
is commonly termed vaporization. The other mode is the ordinary
gradual evaporation which goes on from the _surface_ of the liquid.
This process is going on continuously, not requiring any particular
degree of heat, but occurring under all degrees of the temperature
of a liquid. Its rapidity, however, is in proportion to the degree
of heat, as may be seen by the rise of vapor from water that is
being heated, long before it begins to boil. The same thing can
also be seen in a bright summer's morning, when the heat of the sun
causes the moisture gathered from rain or dew to rise so abundantly
from fences, and boards, and roofs as to be visible like smoke.
287. =Solution of Water in Air.=--Evaporation is constantly going
on from every wet surface, except when the air is so loaded with
moisture that it can take up no more. The vapor is not ordinarily
visible, the particles of water passing quietly upward among those
of the air, being dissolved in the air just as some solids are
dissolved in water. It becomes visible only when so much of it
rises that the solution of the water in the air is not readily
effected. The readiness with which the solution takes place
depends much upon the temperature of the atmosphere. Some very
common phenomena illustrate this. In a very cold day the breath
of animals, as it comes out of the mouth, seems to be loaded with
moisture. Why? It is not because there is more moisture in it than
in warm weather, but because cold air can not hold in solution so
much water as warm air can. The same explanation applies to the
smoking of wet fences and roofs in the sun of a summer's morning.
The moisture is heated by the sun, but the air, not having become
very warm as yet, can not readily dissolve all the moisture that
rises. The phenomenon is not apt to occur when the hot sun shines
after a shower at mid-day or in the afternoon, because then the air
is warm enough to take up all the moisture that is sent up into it.
How water, being heavier than air, rises in the atmosphere is a
mystery. It has been supposed by some that it was owing to a kind
of affinity existing between water and air. But in opposition to
this is the fact that evaporation takes place more rapidly under
the exhausted receiver of an air-pump, where there is almost no
air, than it does where it is freely exposed to the atmosphere.
288. =Clouds.=--The water which goes up in the air in evaporation
is variously disposed of. Some of it is deposited as dew or frost.
Some of it forms fog. Some of it also mounts far upward and forms
the clouds, which are really collections of fog made high up in
the air. In fog and in clouds the water which in its evaporation
is invisible becomes visible. Let us see how this is. There is
always more or less of water in clear air, but the particles are
so minutely divided and so thoroughly mingled with the particles
of the air that they can not be seen. But in a fog or cloud the
particles of water are gathered together in little companies, as we
may express it. And it is supposed, some think ascertained, that
each of these companies of particles is globular and hollow. If
so, then we may regard every cloud as a vast collection of minute
bubbles or balloons careering through the air.
[Illustration: Fig. 202.]
[Illustration: Fig. 203.]
289. =Shapes of Clouds.=--Clouds have a very great variety of
shape, the causes of which are for the most part not understood.
They are generally divided into four classes: Cirrus, Cumulus,
Stratus, and Nimbus. The _Cirrus_ is represented in Fig. 202 (p.
225). It is a light, fleecy cloud, having graceful turns like
curls, and hence its name, which is the Latin word for curl. Such
clouds are commonly very high up in the air. The _Cumulus_ (Latin
for heap) you see in Fig. 203 (p. 225). Clouds taking this form
appear as heaps rounded upward, and often appear like mountains
of snow when they are illuminated by the sun. We see such clouds
mostly in summer. The _Stratus_ (Latin for covering) is seen in
the same figure under the Cumulus. Clouds of this form lie low in
the horizon, stretched along like a sheet. They often form in the
latter part of the day, and increase in the night, but the rising
sun dissipates them. The _Nimbus_, or rain-cloud, is represented
in Fig. 204 (p. 226). It has a uniform gray or dark color. We
often have two forms of cloud mingled together. Thus in Fig. 205
(p. 226) we have a mixture of the Stratus and the Cirrus, termed
_Cirro-Stratus_. This is commonly called the mackerel-sky, and is
quite a sure prognostic of rain. Then we have the _Cirro-Cumulus_,
Fig. 206 (p. 227), and the _Cumulo-Stratus_, Fig. 207 (p. 227).
[Illustration: Fig. 204.]
[Illustration: Fig. 205.]
Water is gathered into clouds undoubtedly, in part at least, from
the influence of attraction. But what the circumstances are that
give them all these various shapes we know not. Whatever they are,
they sometimes operate very extensively, giving a similar shape
to all the clouds that cover the whole arch of the heavens; and
at other times they operate variously in different localities,
producing different shapes, sometimes even in near neighborhood to
each other. Sometimes the edge of a cloud is irregular, or curved,
or feathery; and at others it is a well-defined line, stretching
along over a large portion of the horizon. In all these cases we
have only divers arrangements of the same thing--a collection of
vesicles of water containing air, which is made lighter than the
air outside of the cloud by means which I shall speak of in another
part of this chapter.
[Illustration: Fig. 206.]
[Illustration: Fig. 207.]
290. =Rain, Snow, and Hail.=--When it rains the vesicles or minute
bubbles of which the clouds are composed are broken up, and each
drop of rain contains the water which came from a multitude of
these vesicles. But let us see exactly how this result is produced.
Rain comes from the contraction of the clouds by cold. A cold
current of air coming in contact with a cloud will condense its
bubbles into drops, and these of course will fall. The same result
occurs if a cloud passes into a cold stratum of air. But let us
look at the process more minutely. Let us see what the effect of
cold is upon the bubbles.
[Illustration: Fig. 208.]
The first effect may be made clear by Fig. 208. If a bubble be
contracted by the influence of cold, the water of its wall being
made thicker, there will be a gathering of it from gravitation at
the lower part, as represented by the dotted line. You often see a
similar effect in the soap-bubble. It rises filled with the warm
air from your lungs, and as it goes up it is contracted by the
colder air which is around it. This contraction makes the water
hang downward from the bottom of it. And as the soap-bubble at
length perhaps bursts in the air from the weight of this water, so
it is with the vesicles in the cloud. And many of these, united
together by attraction, form a drop. When the cold is sufficiently
severe it makes the water of the ruptured vesicles of the cloud
arrange itself in snow-crystals instead of drops. And when the cold
acts with great rapidity upon a cloud it presses the particles
of the water together so suddenly that there is not time for the
crystalline arrangement, and hail is formed.
291. =Vaporization.=--The production of vapor by boiling differs in
some respects from quiet evaporation. Here the liquid is raised in
temperature to its boiling point, and the formation of vapor is not
confined to the surface. In water the boiling point is 212°, but it
varies more or less from this in other liquids. Thus the boiling
point of alcohol is 173°, of ether 95°, oil of turpentine 568°,
and mercury 652°.
[Illustration: Fig. 209.]
292. =Influence of Pressure upon the Formation of Vapor.=--Pressure
restrains the production of vapor, whether it be formed by
evaporation or vaporization. We know by experiments with the
air-pump that the less pressure of air there is upon the surface of
a liquid the more rapidly will evaporation from it go on. I have
already spoken of the influence of pressure upon the boiling of
liquids in § 171. I will give here a few additional illustrations.
Ether boils when it is heated to 95°, three degrees below the heat
of the blood in our bodies. If we place some of it in a vessel
under the receiver of an air-pump, by exhausting the air we can
so take off the pressure that the ether will boil at the ordinary
temperature of the air in a room. The restraint of pressure upon
boiling is very strikingly shown in the _digester_, Fig. 209. This
is a strong boiler, _a_, partly filled with water. A thermometer,
_d_, is fastened into it so as to indicate the heat of the water.
There is also a tube, _c_, extending to near the bottom of the
boiler into a small quantity of mercury which is there. Let, now,
the boiler be heated till the water boils, the air being left to
escape by the stop-cock, _b_. If the stop-cock be shut, and we
continue to apply the heat, we can raise the water to a very high
temperature without having it boil at all, because of the pressure
of the condensed steam upon its surface. An apparatus somewhat
after this plan, called _Papin's digester_, has been used sometimes
in cooking. The great heat to which water can thus be raised causes
it to extract the nutritious matter from bones and cartilages,
affording material for soup from what is commonly thrown away. To
guard against the danger of explosion a safety-valve is provided,
having a weight upon it which will keep it shut until a certain
amount of pressure accumulates, and then it is forced open, letting
out some of the steam.
293. =Steam.=--The cloud of steam, so called, which you so often
see escaping from a locomotive is not really steam. Steam is
transparent and invisible. You can see that it is so if you observe
it issuing from the spout of a tea-kettle. It is only after it gets
an inch or more from the spout that it becomes visible, and then it
is really changed from steam into water by the condensing influence
of the cold air. And the water in the cloud thus formed is probably
in the same condition with the water in the clouds above, as
described in § 288.
[Illustration: Fig. 210.]
294. =The Steam-Engine.=--As compressed or condensed air has
great power by its elasticity, as seen in the air-gun, § 164, so
also has condensed steam. It is steam condensed, and endeavoring,
therefore, in proportion to its condensation, to expand itself,
which constitutes the moving force of the steam-engine. The steam
is generated in a boiler, having, like the boiler of Papin's
digester, a valve with a weight attached to it. This valve is
called a safety-valve, because when the steam has reached a certain
degree of condensation it lifts the valve, and, as some of the
steam escapes, such an increase of pressure as would occasion an
explosion is prevented. The expansive force of steam in a boiler
is estimated in pounds by the weight on the valve, and hence the
common expression that there are so many pounds of steam on. But
the boiler is only the generator of steam, and it remains to show
how the steam is used in moving machinery. This is done by allowing
the steam to pass from the boiler into a cylinder, and then move a
piston back and forth by its expansive force. The manner in which
it does this may be made clear by the diagram, Fig. 210 (p. 231).
Let _e_ be a piston in a cylinder, _f_, which has four openings,
_a_, _b_, _c_, and _d_. These all have valves. The steam is
supplied from the boiler to the cylinder through _a_ and _c_, and
makes its escape from _b_ and _d_. Suppose, now, the piston is near
the bottom of the cylinder, as represented. The valve at _a_ is now
opened that steam may enter to push up the piston, and the valve
at _b_ shuts that the steam may not escape. At the same time, that
pressure may be taken off from the upper surface of the piston, _d_
opens that the steam may escape, and _c_ shuts that none may enter.
When the piston is to be forced downward all this is reversed--_c_
opens to admit the steam, _d_ shuts to prevent its escaping; and
below, _b_ is opened to let the steam escape, and _a_ is shut to
prevent any from entering. This is the plan of what is called the
high-pressure engine. The low-pressure engine differs from it in
having the steam, as it escapes from the cylinder, pass into water
to be condensed. The latter requires less pressure of steam to work
it, and therefore is the safest. The manner in which the motion of
the piston is made to work various kinds of machinery I need not
stop to explain, especially as exemplifications of it may be seen
in every quarter.
295. =Communication of Heat.=--Heat has a constant tendency to an
equilibrium. If therefore any warm substance be in the neighborhood
of one which has less heat, a flow of heat from the former toward
the latter takes place. Now this communication of heat occurs in
three different ways, called Convection, Conduction, and Radiation.
I will speak of each of these separately.
296. =Convection.=--This mode of diffusion of heat is in operation
in those substances whose particles are movable among each
other--viz., liquids and aeriform substances. I have already
alluded to examples of this mode in speaking of the movements which
heat causes in these substances. The heat goes along with the
particles which are moved, or is _conveyed_ along with them, and
hence the term convection. In this movement the heated particles
always ascend, for the reason given in § 275. Of the multitude of
examples of convection I will present but a few.
In the upward current about a stove-pipe you have an example
of convection, the heat generated being carried upward by the
particles of this current. This being so, the heat of a stove has
no effect upon the air _below_ it by convection, though it does
have by radiation, as you will soon see. Any hot fluid becomes
cool chiefly by convection. The air coming in contact with it
taking some of its heat rises, and other air comes in its turn
to be also heated, and so on till the fluid becomes of the same
temperature with the air, and then the currents of air cease.
The liquid cools more rapidly by stirring it, because the air is
brought into contact with a greater extent of surface, and so the
heat is conveyed away more rapidly. The result is the same whether
we disturb the surface by stirring it or by blowing upon it. In the
latter case, however, the effect is increased by making the air
to come more rapidly upon the disturbed surface. So in fanning,
it is the bringing of the air faster upon the surface of the body
that causes the more rapid, convection of heat from it. Every one
must have observed the fact that a buckwheat cake cools much more
quickly than a flour or rice cake. It is because it has so many
pores and little projections, and so presents a much larger amount
of surface to the heat-conveying air than the smoother and more
solid cakes. Viscid fluids, as molasses, oil, etc., when heated do
not cool as readily as water, because their particles are not as
movable, and therefore heat is not conveyed as rapidly upward to be
given off to the air.
[Illustration: Fig. 211.]
297. =Conduction.=--In this mode of diffusion the heat goes through
or among the particles of substances. For example, if one end of a
bar of iron be held in the fire, it travels through or among the
particles to the other end. The gradual progress of the heat may
be seen by the following simple experiment: Take a rod of iron and
attach to it, as seen in Fig. 211, some little balls of wood by
means of wax. By heating one end with a lamp the balls will drop
one after another as the heat passing along melts the wax which
holds them.
[Illustration: Fig. 212.]
298. =Conductors and Non-Conductors.=--Heat is conducted more
rapidly through some substances than through others. There is great
variety in this respect. There is considerable among those which
are reckoned as good conductors, as is shown by the experiment
represented in Fig. 212. Here are cones of the same size of seven
different substances--copper, iron, zinc, tin, lead, marble, and
brick--all tipped with a little wax, and placed on a stove. The wax
will melt on the copper cone first, showing that this is the best
conductor of them all; and on the brick one last, showing that this
is the poorest conductor. The conducting powers of the rest are
according to the order in which I have mentioned them.
Those substances which allow heat to pass through them very slowly
are called non-conductors. The term, though convenient, is not a
strictly correct one, for there are no substances which do not
conduct heat in some degree. Wood is one of these poor conductors,
and hence wooden handles are put upon various instruments and
vessels that are used about fires, as the soldering irons of the
tinman, the metallic tea-pot, etc. As cloth is a non-conductor,
the holder is used in taking off the tea-kettle and in using the
flat-iron. Glass is so poor a conductor that if you hold a rod or
tube of it across the flame of a spirit lamp or gas burner, and
heat it even to redness, you can place your fingers very near to
the heated portion with impunity. I had occasion to-day to bend a
small glass tube in this way, and I observed some water in it quite
near to the heated part which remained undisturbed through the
process. It is the non-conducting quality of glass that makes it so
liable to break, when it is thick, if it be exposed to any sudden
change of temperature. For example, if hot water be poured into
a thick glass vessel, the inner surface is quickly expanded; but
the outer surface not expanding with it, because the heat is not
readily conducted through, this irregularity in expansion causes
a fracture. It is for this reason that the flasks, retorts, etc.,
used by the chemist are made very thin, especially where the heat
is to be applied.
[Illustration: Fig. 213.]
299. =Davy's Safety-Lamp.=--One of the most beautiful applications
of the conduction of heat we have in the Safety-Lamp of Sir
Humphrey Davy, an invention which has been the means of saving the
lives of multitudes of miners. It is represented in Fig. 213. With
this lamp one can go into the midst of the most explosive gases
with impunity. Now all that prevents the flame within from setting
on fire the gases without is a covering of wire-gauze. This, being
a good conductor, conducts off the heat of the flame within so
rapidly that it can not go through the openings _as flame_, and
so does not set fire to the gas without. The fact upon which the
construction of this lamp was based was discovered by trying many
experiments. Among them were the following: A piece of wire-gauze
was held over a candle so that its flame struck against it. The
smoke issued above, but no flame. Then a stream of gas was allowed
to pass through the gauze, as seen in Fig. 214, and was set on fire
above. It burned without inflaming the gas below.[4]
[Illustration: Fig. 214.]
300. =Relation of Density to Conduction.=--Generally the more
dense a substance is the better is its conduction of heat. Thus
the metals are better conductors than wood, marble than brick, the
solids than liquids, and liquids than aeriform substances. We have
frequently a good illustration of the difference between stone and
brick as conductors in the melting of snow on sidewalks. If a light
snow fall in the spring, after the earth has become somewhat warm,
you will see it melted from the stone walks much before it is from
the brick ones. This will be especially the case if the snow be
melted mostly by the warmth of the earth without the agency of the
sun. The explanation is obvious. The stone is a better conductor
than the brick, and therefore the heat of the earth comes up
through the former more rapidly than through the latter.
[Illustration: Fig. 215.]
[Illustration: Fig. 216.]
301. =Conduction in Liquids.=--That liquids are poor conductors of
heat may be shown by an experiment or two. If a thin glass tube
closed at one end be filled with water, and the heat of a spirit
lamp be applied to its upper portion, as seen in Fig. 215, though
the water at this portion may be made to boil, there will not be
the least movement in the lower part. This will be very obvious
if you have some amber-dust in the water. Again, let a little
water be frozen in the lower part of the tube by placing it in a
freezing mixture, and introduce a little oil, and then over that
some alcohol. Hold now the tube over the chimney of a lamp, as
represented in Fig. 216, until the alcohol boils. The ice in the
bottom of the tube will not be in the least affected, and the oil
will be but slightly heated. If the heat were to be applied in
either of the above cases at the lower part of the tube the result
would be different, because convection would then operate in the
diffusion of the heat.
302. =Air as a Non-Conductor.=--Heat is rapidly diffused in air by
convection; but it is only when the air is free that this can be
done. When the air is confined in spaces or pores, or among fibres,
heat makes its way through it very slowly, for it can be diffused
through it then only by conduction. The variety of ways in which
air is of service to us as a non-conductor is almost endless. I
will notice some of them.
303. =Double Windows.=--The efficacy of double windows depends upon
the confined air between them. In the case of the single window a
great deal of the heat inside is lost in this way: The warm air
of the room which comes in contact with the window imparts to it
some of its heat, and, being thus cooled and therefore condensed,
passes downward. As this process goes on continually this downward
current by the window is constant. The current outside is in the
opposite direction. The heat imparted to the window is taken up by
the cold air, and as it thus becomes warmer it passes upward. And
this upward current outside is as constant as the downward current
inside. Now nearly all this is prevented by the non-conducting
quality of confined air in the case of double windows. If a pane
were taken out from the upper part of the inner window, and another
from its lower part, the inner window would be of little use,
for then the heat of the air in the room would be continually
diminished by convection, as when the window is single. The warm
air would pass in at the upper opening, and, being cooled, pass
down through the lower one.[5]
304. =Air as a Non-Conductor in the Walls of Buildings.=--The
spaces included between the outer wall of a building and the
plastering inside being filled with confined air, prevent the heat
of the air in the apartments from passing off readily through
the wall. A house built of brick or stone, with the plastering
placed directly upon the inside of the wall, would be kept warm
with difficulty in winter, because the solid wall would so readily
conduct off the heat to the external air. So, also, such a house
would be very warm in summer, because the heat of the sun and of
the external air would be so rapidly communicated to the air of the
house. In this connection I will mention a contrivance to prevent
the spreading of fires in blocks of buildings, which, though very
effectual, is seldom made use of, partly because it occasions some
trouble and expense, and partly because it takes up a little room.
It is this: A small space is left in the division wall between each
two houses from top to bottom, containing, of course, a body of
confined air, that is, if the space be entirely shut in, which is
as essential here as in the case of the double windows. With such
an arrangement the interior of one house may be entirely consumed
without communicating sufficient heat through the confined air to
set on fire the other.
305. =Fur, Hair, and Feathers.=--Animals that live in cold climates
are provided with suitable coverings for their protection.
Quadrupeds, for example, are covered with fur, and birds have an
abundance of downy feathers. These coverings have no warmth in
themselves, though in common language we speak of them as being
warm. They are simply non-conductors, and so prevent the heat which
is made in the body of the animal from escaping as fast as it
otherwise would. But why are they non-conductors? It is not because
the substance of which they are made is a non-conductor, but
because among their numberless fibres is partially confined that
great non-conductor, air. Let the fur or down be condensed into
a thin hard plate upon the animal, and it would prove of little
service as a protection against cold. Down is much more abundant
on the birds of cold climates than on those in warmer regions,
because more air can be confined among the fibres of down than
among those of common feathers. Quadrupeds that are natives of warm
climates generally have hair instead of fur. When therefore the
horse is taken to a cold climate he requires in winter the defense
of a blanket; and the ox needs under the same circumstances to be
better housed than he ordinarily is. As the elephant is a native
of a climate positively hot, his hairs are scanty and coarse.
Formerly there were elephants in the cold regions of Siberia, as
has been ascertained by remains found there. But the elephant of
Siberia had under its hair, close to the skin, a fine wool to
protect it against the cold. Animals that live in cold climates
have their coverings become finer in fibre in the cold season of
the year, to give them the additional protection which they then
need. And animals with a furry covering, if they are carried into
a warm climate, have their fur become coarse, and approximate the
condition of hair.
306. =Clothing.=--Man has no covering to guard him against cold,
because he is capable of contriving clothing suitable to the
various degrees of temperature to which he may be exposed. The
object of clothing is not to make the body warm, but to keep it so.
The heat of the body is generated continually within itself, and
under all circumstances this heat is maintained quite uniformly at
98°. This, you see, is a much higher degree than the atmosphere
ordinarily has. We are all the time, then, giving off heat to
the air around us, except when the air gets up to 98°. We are
comfortable only when we are giving off heat to a considerable
amount, for the point of temperature which is most agreeable to us
when we are at rest is 70° or a little less, that is nearly thirty
degrees below the temperature of our bodies. When the temperature
is below this we need extra clothing. In making choice of clothing
for various degrees of temperature we practically apply the
principles which I have developed. Those articles of clothing which
can confine or entangle, as we may say, the largest quantity of
air among their fibres are the best non-conductors, or, in common
language, are the warmest. So, too, loose clothing is warmer than
tight, on account of the amount of air between the clothing and
the body. Thus a loose glove is much warmer than a tight one. The
same general fact is exemplified in the coatings of straw which we
put upon tender trees and shrubs in winter. It is the air that is
confined in the tubes of the straw which makes these coverings so
effective a defense. It is probably the air in the pores of the
brick which makes it a poorer conductor than stone, as illustrated
by the fact stated in § 300.
307. =Cocoons.=--Many insects pass through their pupa or transition
state in cocoons. When this is done during warm weather, as in
the case of the silk-worm, the cocoon is simple. But when the
pupa state lasts through the winter special provisions are made
in the arrangements of the cocoon to guard the insect against the
cold. I will cite as an example the cocoon of one of our largest
moths, the Cecropia. This cocoon, fastened to some shrub, keeps its
inmate secure from the rigors of the winter by a very beautiful
arrangement. The real cocoon is similar to that of the silk-worm;
but it has a very dense air-tight outer covering, and the space
between these two coverings of the pupa is filled with a loose
substance, which has air, of course, mingled with its fibres, and
acts, therefore, the part of a blanket for the insect.
308. =Buds of Plants in Winter.=--In the latter part of summer buds
are formed on trees and shrubs, and these contain the germs of the
branches, leaves, and flowers which are to come out the next year.
These of course must be guarded against the cold of winter, and
it is done very much as the pupa is guarded in the cocoon. Each
bud you can see has a covering of scales which is air-tight, and
inside of this there is a soft downy substance, the blanketing of
the bud. In these coverings, which have been called by some one the
"winter-cradles" of the buds, the infant vegetation of another year
rocks back and forth in the wintry winds secure from the cold, till
the warm sun of spring wakes its hidden life into activity.
309. =Snow a Protection to Plants.=--Snow is a good blanket to
the earth, keeping its warmth from escaping into the cold air.
This is because it contains mingled with its feathery crystals
such a quantity of air. If snow come early, before the ground
and the plants in it have become frozen, it will keep them from
freezing through the winter, if it remain during all that time. It
is curious to observe the peculiar arrangement of the snow in the
arctic regions for the preservation of vegetation. First in the
autumn come soft light snows covering up the grasses, and heaths,
and willows. Then as winter advances there are laid on top of these
the denser snows, making a compact, stout roof over the lighter
snows in which the scanty but precious vegetation of those regions
is imbedded. On top of this roof are deposited the snows of spring.
As these melt the water runs off from the icy roof down the slopes,
leaving untouched the plants underneath, which lie there alike
secure from the rush of waters and from the nightly frosts until
the season is sufficiently advanced to bring them out with safety
from their concealment. Then the icy roof melts, and with it the
light snows that have so long encircled the plants, and the sun
wakes them from their long sleep to a new life.
310. =Influence of the Conduction of Heat on Sensation.=--If you
place your hand upon fur hanging at the door of a fur-store it
does not feel as cold as the wood from which it hangs, and the
wood does not feel so cold as the iron bar of the shutter close
by. Why is this, when these substances are exposed to the same
atmosphere, and really have the same temperature? It is because the
iron conducts the heat from your hand more readily than the wood,
and the wood more readily than the fur. So the iron handle of a
wooden pump feels colder than the pump, and the pump colder than
the snow around it. For the same reason, in a cold room the rug or
the carpet will not feel as cold as the poker and the hearth. If
water has stood long enough in a room to be of the same temperature
with the air of the room your hand will feel colder in the water
than in the air, because the water is the better conductor. So much
for the sensation of _cold_. On the other hand, when substances
are so heated as to give us the sensation of heat, the conductors
do this more than the non-conductors. As they receive heat readily
they also readily impart it. For this reason, with a brisk fire the
hearth-stone feels very hot, while the rug before the fire does not.
311. =Radiation of Heat.=--Every substance sends heat into space
constantly in straight lines in every direction. These lines are
radii, and hence the term radiation is applied to heat diffused in
this way. It is very obvious in regard to the sun that it radiates
heat in all directions. The same can be seen in the case of a
heated iron ball. In whatever direction you hold your hand, above,
below, or laterally, you feel the heat. And it makes no difference
whether the ball be red-hot or not. That is, heat is radiated
either with or without light. When a room is warmed by a furnace
it is warmed altogether by convection; but when it is warmed by a
fire, either in a fire-place or a stove, we have both convection
and radiation. The heat which we receive from the sun comes
altogether by radiation.
312. =Connection between Heat and Light.=--The heat and light of
the sun pass together through transparent substances, as air,
glass, water, etc., without heating them to any extent. Thus, when
the heat is transmitted through a lens, § 272, the lens is little
heated, that is, it lets almost all the heat pass through it. The
air is heated by the sun, but not directly to any amount. It is
heated indirectly in this way: the rays of the sun passing through
the air heat the earth, and then the air receives a part of this
heat from the earth, which is diffused through it by convection.
It is otherwise with heat that comes from a common fire. It does
not seem to be so thoroughly united with the light, and therefore
readily parts company with it, as we may say. While the heat and
light of the sun go together through all transparent bodies the
heat of a fire will not go with its light through all of them. So
while the heat of the sun does not warm the glass through which it
passes the heat of a fire will warm it, and therefore glass is an
effectual screen against it. In some operations in the arts a mask
of glass is sometimes worn to ward off the heat. The connection
of light and heat will be farther noticed when I come to treat of
light.
313. =Relation between Radiation and Absorption.=--All surfaces
that radiate will absorb also equally well the heat that is
radiated upon them. All rough and dark surfaces both absorb and
radiate freely; but all light-colored and polished surfaces do both
slowly. For this reason the black, rough tea-kettle is well fitted
to heat water in; but it is not fitted to retain the heat in the
water. On the other hand, the bright, polished tea-pot absorbs heat
poorly, but retains it well.
[Illustration: Fig. 217.]
314. =Reflection of Heat.=--Radiated heat is reflected; and here,
as in the case of motion, § 206, and of sound, § 260, the angles of
incidence and reflection are equal. Some interesting experiments
in relation to the reflection of heat can be tried with concave
metallic mirrors. Thus, if we take two such mirrors, as represented
in Fig. 217, and place in the focus of one a thermometer, and in
the focus of the other a small flask of hot water, or a heated
iron ball, the mercury in the thermometer will rise, although the
mirrors may be many feet apart. Observe how the effect is produced.
Rays of heat go from the flask directly toward the thermometer, as
represented by the lines in the figure; but that the effect does
not come from these can be proved by removing the mirrors, leaving
the flask and thermometer just as they are. When the experiment is
tried in this way no effect is produced on the thermometer, because
it is too far from the source of heat, the flask, to receive any
perceptible influence in this way. The effect comes from the rays
of heat which go to the mirror near the flask, and are reflected
to the other mirror, and then are reflected upon the thermometer,
all of which is represented by the dotted lines. There is another
way, besides that already mentioned, of showing that it is not
the _direct_ rays that produce the effect. After arranging the
apparatus put a screen between the thermometer and the mirror near
it, and the effect will be prevented because the reflection is cut
off. If a piece of ice be substituted for the flask of hot water
the thermometer will fall--an effect opposite to that produced
in the previous experiment. This would seem to show that cold is
radiated, but as there is no such thing really as cold, § 270,
the effect must be attributed to the radiation of heat from the
thermometer to the ice. If a hot ball be placed in the focus of
one mirror and a piece of phosphorus in that of the other, the
phosphorus will be set on fire, though the mirrors may be twenty or
more feet apart.
[Illustration: Fig. 218.]
The reflection of heat may be exhibited very prettily with the
experiment represented in Fig. 218. A sheet of bright gilt
paper is rolled up in the shape of a funnel, with the metallic
side inward. Holding the larger end toward a fire, the rays of
heat coming from the fire into the funnel are reflected toward a
central line, and so pass out of the smaller end of the funnel
concentrated. If, now, a bit of phosphorus or a lucifer match be
held a little distance from this end of the funnel it will be set
on fire.
315. =Formation of Dew.=--It is by the radiation of heat that
dew is formed. The earth is constantly radiating heat into space
as well as the sun. In the daytime it receives a great deal more
than it radiates. But at night this is reversed, and the earth is
cooled. The cooled earth condenses the moisture in the air which
is in contact with it, and so the moisture is deposited. If the
weather be very cold this is frozen, and then we have frost instead
of dew. You observe that the dew does not _fall_, though this is
the ordinary expression. Its formation is analogous to the deposit
of moisture which we so often witness in a hot day in summer on the
outside of a tumbler containing cold water. As the cold tumbler
condenses the moisture in the air, so does the earth at night,
it being cooled by radiation, condense the moisture which has
accumulated in the air by evaporation during the heat of the day.
There are some circumstances which have an influence upon the
deposition of dew and frost. Less is deposited under a tree than
outside of it, because all the heat which radiates vertically
upward from under the tree is radiated back again by the tree.
Hence the efficacy of a covering over plants as a defense against
frost. Clouds operate in the same way, and for this reason no dew
or frost is deposited in a cloudy night. Neither is any deposited
in a very windy night, because the moving air promotes evaporation,
and thus prevents the accumulation of moisture.
Dew is deposited in different amounts on different substances.
This is owing to a difference in radiation. Grass and leaves
radiate heat better than earth, and earth better than stone; and
therefore while stones and gravel-walks may be dry or nearly so,
the loose earth may be moist and the grass and leaves thoroughly
wet. So you see that not even the dew, plentiful as it is, is
wasted by the Creator, but is deposited just where it is wanted to
refresh the parched earth and its vegetation.
316. =Gideon's Fleece.=--If you lay a fleece of wool upon the
ground, it is so poor a radiator of heat that no dew will be
deposited upon it, although there may be an abundance of it on the
grass and leaves in its neighborhood. But this was reversed in the
case of Gideon's fleece. The laws of nature were set aside, and the
fleece was wet with dew while all around was dry.
317. =Dew-Point.=--What is called the dew-point of the air is that
degree of temperature to which any substance must be brought down
in order to have dew deposited upon it. This depends upon the
amount of water there is in the atmosphere. The more there is the
higher is the dew-point. When water condenses on a cold tumbler in
a hot day there is much more water in the air, and the dew-point
is higher, than when no moisture is condensed upon the tumbler.
So after a very hot clear day the earth needs not to be much
cooled to produce a deposit of dew, because the air has become so
highly charged with moisture through the evaporation of the earth
under the hot sun. We can very readily at any time ascertain the
dew-point. Take a glass of water, and, having a thermometer in
it, drop into it some pieces of ice, and watch the outside of the
glass. As soon as it begins to be dimmed with moisture look at the
thermometer, and you have the dew-point.
318. =Freezing Mercury.=--Mercury can be frozen by radiation when
the cold is excessively severe, although the thermometer may
indicate a temperature considerably above -39°, the degree at
which mercury freezes. Suppose that in a clear, still night the
temperature of the air is at -20°. In order to freeze the mercury
it must be cooled down 19 degrees below this. Now this can be
done by surrounding the mercury with some good non-conductor, as
charcoal. This cuts off the supply of heat to the mercury, while
it is all the while giving off heat into space by radiation. In
like manner can ice be formed in an atmosphere that is above the
freezing point, and this is often done in warm climates.
319. =Latent Heat.=--You have seen, § 270, that our sensations do
not inform us accurately of the amount of heat in any substance.
The same is also true of the thermometer. This only indicates the
_sensible_ or free heat. There may be a great deal of heat locked
up, as we may say, in the substance, which can be brought out or
made free by some change in the substance. This heat thus locked up
is called _latent_ heat.
[Illustration: Fig. 219.]
320. =Capacity for Heat.=--The more heat a substance can take in
and render latent the greater is its capacity for heat, as it is
expressed. Thus water has a much greater capacity for heat than
mercury. This can be proved by various experiments. Thus, if we
take two vessels just alike, and having, the one a certain quantity
of water in it, and the other the same quantity of mercury, and
expose them to the same degree of heat, it will take much longer
to raise the water to any specified temperature than the mercury.
Why is this, when they are both receiving the same amount of heat?
It is because the water renders a much larger portion of the heat
latent than the mercury does. We can reverse this experiment.
Take these same vessels with their contents raised to the same
temperature, as indicated by the thermometer, and allow them to
cool in the air side by side. The mercury will cool much faster
than the water, because it has much less of latent heat to part
with. The difference in capacity for heat between water, oil, and
mercury may be shown by the experiment represented in Fig. 219. A
pound of water is put into one Florence flask, a pound of oil into
another, and a pound of mercury into a third. They are all heated
to 212°, and are then placed in funnels filled with pounded ice,
the funnels resting in glass jars of the same size. Now in cooling
these fluids down to a certain point, say 32°, different amounts
of the ice will be melted, in the proportions of 100 and 50 and
3. This shows the proportions of latent heat in them which become
sensible or free as their temperatures are lowered.
[Illustration: Fig. 220.]
321. =Relation of Latent Heat to Density.=--The more dense a
substance becomes the less is its capacity for heat. The heat
produced by hammering iron is the latent heat rendered free by
condensation, this lessening the capacity of the iron for heat. The
same thing can be better illustrated in the condensation of a very
compressible substance, as air. In Fig. 220 you have represented a
glass syringe with a closed end. If there be placed in this end a
little bit of cotton wool moistened with ether, and the piston be
forced downward very quickly, the ether will be set on fire. This
is because the compression of the air lessens so much its capacity
for heat that a great deal of its latent heat is made sensible or
free. The heat which is concealed in it in its ordinary state is,
as we may say, fairly squeezed out, as you would squeeze out the
water that is concealed in the interstices of a sponge.
322. =Coldness of Air at Great Heights.=--You learned in § 152 that
the atmosphere is thinner the farther you go from the earth. It is
very thin, therefore, on the summits of high mountains. This is the
chief reason why it is so cold there, for the rarer the air is the
greater is its capacity for heat, and the more of sensible or free
heat therefore can it render latent.
323. =Relation of Latent Heat to the Forms of Substances.=--Whether
a substance shall be in the form of a solid, liquid, or gas depends
upon the amount of heat which is latent in it. If you take a piece
of ice and melt it in a vessel, the ice and the water in the vessel
that comes from the melting of the ice are both at 32° until the
ice is all melted. But all this time heat is being communicated to
the ice and water. What becomes of it? It is all taken up by the
ice as it changes from its solid to its fluid state, and becomes
latent in it. In fact _every particle of ice must have just so
much of latent heat in order to become fluid_. So, also, if water
be heated to the boiling point, 212°, and be kept boiling, the
water will remain at that point till it is all vaporized. All this
time the water is receiving heat, which, instead of raising its
temperature, is becoming latent in the particles as they change
from their liquid to their vaporous state. As I said of the change
from the solid to the liquid state, so here, _every particle of
the liquid must have just so much of latent heat in order to
become aeriform_. Whenever therefore any solid substance becomes
liquid, or liquid becomes aeriform, heat is absorbed and becomes
latent. So, on the other hand, whenever any aeriform substance
becomes liquid, or liquid becomes solid, latent heat is given
out, and becomes free and sensible. The freezing of water, then,
is a source of warmth to the air in its neighborhood--a fact which
is practically made use of when tubs or pails of water are placed
in conservatories to keep plants from freezing; and the thawing
of snow and ice is a source of cold, as is exemplified by the
chilliness of the air occasioned by this process.
324. =Clouds and Latent Heat.=--The water of which clouds are
composed is heavier than air. Why, then, does it remain suspended?
Why is it necessary that it should be collected into drops in order
to have it fall? This question can be answered by looking at the
manner in which clouds are formed. A cloud, I have stated in § 288,
is made up of minute vesicles or bubbles containing air. Now the
air in these bubbles is lighter than the air that is around the
cloud, because it is warmer. But how does it get its heat? In order
to understand this observe what the bubble is made from. It is made
from the water which was in the air in a state of vapor, or in its
aeriform state, for this is the state of water that is evaporated
and dissolved in the atmosphere. But when it forms the vesicle it
goes out of this state and becomes a liquid, for the wall of the
vesicle is a liquid wall, just as the wall of a soap-bubble is. Now
in passing out of the aeriform into the liquid state some latent
heat must be made sensible. What becomes of this sensible heat? It
just heats the air in the vesicle, and so makes it like a heated
air-balloon. So all clouds are collections of innumerable heated
air-balloons, and the reason that some clouds are higher up than
others perhaps is that their balloons have warmer and therefore
lighter air in them.
325. =Freezing Mixtures.=--The intense cold produced by these
mixtures is the result of the change of free or sensible heat into
latent. For example, when salt and snow are mingled together a
melting of the two is quickly produced. In this sudden change of
a solid into a fluid a great quantity of heat must be rendered
latent, and therefore there will be a great loss of sensible heat
by whatever the freezing mixture comes in contact with. The process
here, you see, is the opposite of solidification in relation to
latent heat. A portion of the snow, after melting with the salt,
becomes solid ice. Why is this? It is because it gives up its
sensible or free heat to portions of the snow that are in the
process of melting and are therefore making heat latent.
326. =Cold from Evaporation.=--If you pour a little ether into the
palm of your hand it will rapidly disappear in vapor, producing a
very cold sensation. This sensation occurs because, in the change
of the liquid into the vaporous or aeriform state, some of the
sensible heat of your hand is abstracted to become latent in the
vapor. The evaporation of water also produces cold, though not
as decidedly as ether, because its change into vapor is not so
rapid at ordinary temperatures. We make a practical use of the
evaporation of water in many different ways. Thus we sprinkle water
in a hot day upon the floors of piazzas, steps, etc., that the
evaporation may make much of the sensible heat about our houses
latent. For the same purpose, in hot climates, apartments are often
separated from each other by mere curtains, which are occasionally
sprinkled with water. So the inhabitants of such climates often
cool their beverages by keeping a wet cloth for some time wrapped
around the vessels that contain them. Evaporation is an important
remedy for many cases of disease. For example, if the head be
hot, a steady application of a wet cloth to the forehead, though
a simple remedy, is generally effectual, and sometimes is very
important. Most people make the application in a wrong manner. They
put on several thicknesses of cloth, when a single thickness is the
best, because it will best secure the evaporation, which is the
cause of the relief afforded.
[Illustration: Fig. 221.]
327. =Freezing in the Midst of Boiling.=--It is from the quantity
of heat rendered latent by evaporation that water can be frozen in
the midst of boiling ether; and, paradoxical as it may seem, the
boiling of the ether is the cause of the freezing. The experiment
is performed in this way: Place a test-tube or a little thin vial
with water in it in the midst of some ether in a shallow vessel
under the receiver of an air-pump. On exhausting the air the ether
will boil, evaporation taking place rapidly because the pressure
of the air is taken off from the ether. Now as the ether passes
into vapor it extracts so much free heat from the vial of water
that the water is cooled down to the freezing point, and so becomes
solid. Water can be frozen even by its own evaporation. It is
done in this way: Let a shallow vessel, _b_, Fig. 221, contain
a little water, and the vessel _c_ oil of vitriol or sulphuric
acid. When the air is exhausted, the pressure of air being taken
off from the water, vapor rises from it freely. As the sulphuric
acid has a great attraction for water it absorbs this vapor, and
so vapor continually rises from the water the more rapidly because
what is formed is absorbed, instead of remaining to make pressure
on the water. The result is that this rapid formation of vapor,
requiring that a great quantity of heat should be made latent, at
length abstracts so much heat from the water that remains that this
becomes solid.
328. =Degree of Heat Endurable by Man.=--It was formerly believed
that the human body could not endure with impunity, even for
a short time, a much higher degree of temperature than that
which is met with in hot climates. But in the year 1760 it was
accidentally discovered that a much higher temperature than this
could be endured. An insect was destroying at that time the grain
gathered in some parts of France, and it was found that if the
grain was subjected to a certain high degree of temperature the
insect was killed, and yet the grain was not injured. In trying
some experiments in regard to this matter the experimenters wished
to know the point at which the thermometer stood in a large
oven. A girl attending on the oven offered to go in and mark the
thermometer. She did so, remaining two or three minutes, and the
thermometer was at 260°, that is, 48° above the boiling point of
water. As she experienced no great inconvenience from the heat she
remained ten minutes longer, when the thermometer rose to 76° above
that point. These facts were published, and prompted scientific
men to try other experiments. In England, Dr. Fordyce, Sir Charles
Blagden, and others, went into rooms heated even to 240° and
260°, and remained long enough to cook eggs and steaks, and yet
themselves suffered little inconvenience. The pulse was quickened,
the perspiration was very profuse, but the heat of the body, as
ascertained by putting the thermometer under the tongue the moment
they came out, was scarcely raised at all. The air in which they
were roasted eggs quite hard in twenty minutes, and when it was
applied by a pair of bellows to a steak it cooked it in thirteen
minutes. The question arises, how is it that this high degree of
heat did not produce more effect upon the body? One reason is that
the heat of the air in the immediate neighborhood of the body was
continually reduced by the evaporation of the free perspiration,
sensible heat being thus converted into latent. Another reason
is that the air is not a good conductor, and therefore did not
communicate its heat readily to the body. Dr. Fordyce and his
friends found that they could not touch with impunity any good
conductor, as the metals, and they were obliged to wear upon their
feet some non-conducting substance.
329. =Formation of Ice.=--Before dismissing the subject of heat
I must notice the grand exception which we have to some of the
operations of heat in the formation of ice. Heat generally produces
expansion. But in the case of water this law of expansion is set
aside, and the reverse is established. This is done, however, only
within a small range of temperature, viz., from the freezing point
up the scale about seven degrees. In all degrees above that the
usual expansion by heat takes place. The exception occurs at this
part of the scale for a special purpose, viz., _that water, in
distinction from other substances, shall become more bulky, and
therefore lighter, as it takes the solid form_.
[Illustration: Fig. 222.]
330. =Description of the Process of Freezing.=--In order to make
the process of freezing clear to you I will describe it as it
ordinarily occurs, that is, from the action of cold air upon the
surface of water. The uppermost layer of the water imparts some of
its heat to the air in contact with it. This air rises and colder
air takes its place, which being warmed in its turn rises to make
way for more of the cold air. You have therefore a constant current
of warmed air upward from the water. In the mean time there is a
current of a different character in the water--a downward one.
As fast as the water at the surface parts with heat to the air
it falls, other warm water taking its place, to cool in its turn
and go down. This falling of the cooled water goes on regularly
until a portion of water becomes cooled down to 39°, that is, 7°
above the freezing point. This layer does not sink, but remains
at the surface, for it is lighter than the warmer water below.
This is because the law that heat expands matter is now reversed.
Beyond this point of the thermometer the colder the water is the
lighter it is. As the cooling now goes on from the air coming, as
before, in successive layers to the water, the cooled water at
the surface continually increases. At first it is a mere single
layer of particles, but after a while it is quite a body of cold
water lying on the warmer water below. At length some of it is
cooled down to 32°, the freezing point, and a thin film of ice
now forms. The state of things just at this stage of the process
may be represented by a simple diagram, Fig. 222. Let the line
_a_ represent the film of ice. The space between _a_ and _b_ is
the portion of water cooled down below 39°. The space below _b_
is occupied by the water which is above this temperature. In the
space between _a_ and _b_ the cooler the water is the nearer it
is to the surface. That is, from the line _b_, where the water is
exactly at 39°, as you go upward, the water lessens in temperature,
it being successively 38°, 37°, 36°, etc., till, just in contact
with the film of ice, _a_, it is at 32°. The ice goes on to thicken
gradually by additions below. But it is to be remembered that ice
is a good non-conductor, so that the very first layer of ice makes
the cooling of the water proceed more slowly than before. And the
thicker the ice becomes the slower is the cooling. This secures
against too great a formation of ice.
331. =Why the Above Exception to Expansion by Heat Exists.=--That
we may see the reasons in part for the grand exception to the
general law of expansion by heat which I have illustrated, let us
see what would be some of the results if the exception did not
exist. In that case the process of freezing would be as follows:
The water would communicate its heat from the surface to the air,
as before described, and there would be a constant downward current
of the cooled water. When any portion of the water became cooled
by the air down to 32°, it would become ice, and would sink to the
bottom. And after the process of freezing had once begun, there
would be a continual accumulation of ice at the bottom so long as
the air remained cold enough to cool the water with which it comes
in contact down to 32°.
The result may be stated in the general thus: Freezing would not
begin so quickly as it now does; but when once begun it would prove
very destructive. It would not begin as soon, because the whole
of any body of water must be cooled down to just this side of 32°
before it could begin. This would not take long where the water
is shallow, but it would where it is deep. All shallow bodies of
water, then, would be frozen up quite early in the winter; and as
water is a poor conductor, and thawing must go from above downward,
some of them would not be thawed out again fully till quite into
the next summer, if even then. And where the water is quite deep
ice would at length begin to form, and when formed it would be
exceedingly slow in thawing. In some cases it would never be thawed
with such a body of non-conducting water to guard it against the
warmth above. It is easy to see that the heat of spring and summer
would not thaw out any thing like the quantity of ice that it now
does. The reign of ice and snow on our earth would therefore be
vastly more extensive than now, and what is worse, it would be
extended more and more every year. Under such circumstances there
would be great destruction of both animal and vegetable life. I
will mention, however, but a single item, as it would occupy too
much space to go into this subject with any fullness. In the water
under the ice, which is always above 39°, except that which is
close to the ice when freezing is going on, there is a vast amount
of busy life which would be destroyed if ice were formed at the
bottom, chilling all the water above.
332. =Why the Freezing Point is at 32°.=--If the freezing point
of water were higher than 32°, freezing would occur so early in
the autumn, and the ice and snow would last so late in the spring,
that the season would be too short to raise our supplies of fruits
and grains. If, on the other hand, it were at a lower point, the
earth would not have the protection of its light coat of snow, but
instead would be chilled by rains so cold that barrenness would
be the result. The multitudes of animals, too, that now live so
securely in the water, some of them even with the ice above them,
would all perish with the cold.
333. =Force of Expansion in Ice.=--As ice occupies one seventh
more room than the water from which it is formed, it exerts in its
formation an expansive force which under various circumstances
produces varied and often remarkable results. Of the many
experiments which have been tried to show the force of this
expansion I will mention but one. A bomb-shell was filled with
water, at Montreal, and closed with an iron plug which was driven
in with great force. The plug was thrown a distance of 400 feet by
the expansion when the water froze. This expansion is sometimes
an inconvenience to us, as in bursting water-pipes; but besides
the great service which it does in the earth, already noticed,
it is of service also in loosening the soil, and in supplying it
with requisite ingredients from the rocks by breaking them up and
pulverizing them in small quantities from year to year.