If some water is poured into a saucer and is allowed to stand even in a
cool room or in the open air, you know that it sooner or later
disappears. Wet clothes hung on a line soon dry—that is to say, the
water clinging to them disappears or =evaporates=. The disappearance of
the water under these circumstances results from the property just
mentioned. In fact, it becomes gaseous water of the density appropriate
to the temperature, and as such mixes with the air as any other gas
would do. And as the sea, lakes, and rivers, are constantly giving off
gaseous water into the air in proportion to the temperature, it is not
wonderful that the atmosphere always contains gaseous water.
Air is said to be moist when the weight of water in a given quantity,
say 100 cubic inches, is as much, or nearly as much, as can exist in the
state of gas at the temperature. Under these circumstances, if the
temperature is lowered even a very little, some of the gaseous water is
converted into liquid water. We see this in hot moist weather, when the
outside of a tumbler of fresh drawn cold spring water immediately
becomes bedewed. The gaseous water in immediate contact with the
tumbler, in fact, is cooled down below the point at which it can all
exist as gas, and the superfluity is deposited as dew. In such days wet
clothes do not dry well, because there is, already, nearly as much
gaseous water in the atmosphere as the amount of heat marked by the
thermometer can maintain in that state.
39. =When Hot Water is cooled, it Contracts to begin with, but after a
time Expands.=
We have now seen what a wonderful change is brought about by heating
water. At first, it expands gradually and slightly; but, when it reaches
the boiling-point, it suddenly expands enormously, and is no longer a
liquid, but a gas.
On the other hand, if warm water is allowed to cool, it gradually
contracts till it reaches the ordinary temperature of the air in mild
weather; but, if the weather is very cold, or if the water is cooled
artificially, it goes on contracting only down to a certain temperature
(39°), and then begins to expand again. In this peculiarity water is
unlike all other bodies which are fluid at ordinary temperatures. Hence
the temperature of 39° is that at which pure water has its greatest
density or specific gravity, and water at this temperature is heavier,
bulk for bulk, than the same water at any other temperature. Therefore
if water at the top of a vessel is cooled down to this temperature, it
falls to the bottom, and if the water at the bottom of a vessel is
cooled below this temperature it rises to the top.
40. =Water cooled still further becomes the transparent brittle solid
Ice.=
Our tumbler of water, if put out of doors on a cold winter’s night,
would gradually cool until it assumed a temperature of 39° throughout.
Cooling below this temperature, the water so cooled would gradually
accumulate at the surface by reason of its less density, and its
temperature would fall till the thermometer placed in it marked 32°. As
soon as this upper water cooled ever so little below 32°, a film like
glass would form on its surface by the conversion of the coldest fluid
water into solid water or =ice=. And if all the water cooled down to the
same degree it would all gradually change into the same kind of
substance.
In this condition water is solid. It occupies space, offers resistance,
has weight and transmits motion as the water did, but if you shake it
out of the tumbler in a cold place it retains its form without the least
change. If you press it, it proves to be exceedingly hard and
unyielding; and, if the pressure is increased, it becomes crushed and
breaks like glass. It may thus be crushed to powder, and the ice powder
can be formed into heaps as if it were sand.
Just as any quantity of steam has exactly the same weight as the water
which was converted into it by heat; so the ice has exactly the same
weight as the water which has been converted into it by taking away
heat.
41. =Ice has less Specific Gravity than the Water from which it was
formed.=
But though the ice in the tumbler has the same weight as the water had,
it has not the same volume. The expansion which began at 39° goes on,
and when water passes into the solid state its volume is about 1/11th
greater than it was at 39°. Taking water at this temperature as 1·0, ice
has a specific gravity of 0·916.
But although water in freezing expands only to this small amount, it
resembles steam in the tremendous force with which it expands. If you
fill a hollow iron shell quite full of water, screw down the opening
tight, and then put it in a cold place where the water may freeze, the
water as it freezes will burst the iron walls of the shell. You know
that when the winter is severe, the pipes by which water is brought to a
house often burst. This is because the water in them freezes, and, being
unable to get out of the pipe, bursts it, just as you may burst a jacket
that is too tight for you by stretching yourself. Among the bare
hill-tops, or on the face of cliffs exposed to the weather, the
strongest and hardest rocks are every winter split and broken, just as
if quarrymen had been at work at them. In the summer the rain-water gets
into the little cracks and rifts in the stone and lodges there. Then the
winter comes with its cold and freezes the water. And the water bursts
the rocks asunder just as it bursts our waterpipes.
42. =Hoar Frost is the Gaseous Water which exists in the Atmosphere,
condensed and converted into Ice Crystals.=
In the winter-time you often notice, on a clear sharp night, that the
tops of the houses and the trees are covered with a white powder called
=hoar frost=; and, on the windows of the room when you wake up, you see
most beautiful figures, like delicate plants. Take a little of the
hoar-frost, or scrape off some of the stuff that makes the window look
like ground glass, and you find that it melts in your hand and turns to
water. It is in fact ice. And if you look at the figures on the window
pane with a magnifying glass you will see that they are made up bits of
ice which have a definite shape, and are arranged in a regular pattern.
Each of these definitely shaped bits of ice has been formed in the
following way. The air in the room is much warmer than that outside, and
there is mixed with it nearly as much water, derived from the breath and
the evaporation of moist surfaces, as can maintain itself in the gaseous
state at the temperature. The windowpanes, being thin, are cooled by the
outside air, and of course the gaseous water inside the room, when it
comes in contact with the cold windowpanes, becomes condensed on them
into fine drops of cold water. The panes becoming colder and colder,
these minute drops at last freeze, and the water not only becomes solid,
but it =crystallises=; that is to say, the little solid masses take on
more or less regular geometrical forms with flat faces, inclined to one
another at constant angles, so that they resemble bits of glass cut
according to particular fixed patterns. All ice is in fact crystalline,
but in ice which has been formed from thick sheets of water, the
crystals are so packed together that they cannot be distinguished
separately.
43. =When Ice is warmed it begins to change back into Water as soon as
the Temperature reaches 32°.=
A lump of ice brought out of the open air in very cold weather may have
a temperature of 30°, or 20°, or lower. If such a lump is brought into a
warm room it gradually becomes warmer, but remains unchanged otherwise,
until it has risen to 32°. Then it begins to melt, and remains at 32° as
long as it is melting; and the water which proceeds from it is at first
also at 32°.
If you were to throw a lump of ice into the middle of a hot fire, so
long as a particle of ice remained as such, it would have a temperature
of 32° and no more. This is a fact exactly parallel to that which is
observed when water is raised to the boiling-point. So long as any of
the water remains unconverted into steam it becomes no hotter. Moreover
the steam itself is at first at 212°.
44. =Ice the solid, Water the liquid, and Steam the gas, are three
states of one natural object; the Condition of each State being a
certain Amount of Heat.=
Ice, liquid water, and steam, are three things as unlike as any three
things can well be. What do we mean then by saying that they are states
of one substance, water?
What we really mean is that if we take a given quantity of water, say a
cubic inch, and change it first into ice and then into steam, there is
something which remains identically the same through all these changes.
This something is, in the first place, the weight of the material
substance. The water weighs 252½ grains, the ice into which it is
converted weighs 252½ grains, and the steam produced from it weighs 252½
grains. In the second place, the same force would cause the ice, the
water, and the steam, to move with the same rapidity; and, when set in
motion, they would produce the same effect upon anything movable against
which they struck.
In the third place, when you study chemistry, you will learn that the
ice, the steam, and the liquid water, would yield the same weight of the
same two gases, =oxygen= and =hydrogen=, and nothing else. Every one
cubic inch of water, 1,700 cubic inches of steam, and 1/111 cubic inch
of ice, yield 281/18 grains of hydrogen, with 2248/18 grains of oxygen,
and nothing else. (See § 50.)
As there is not the slightest difference in weight between a given
quantity of water and the ice, or the steam, into which it may be
converted, it is clear that the heat which is added to or taken from the
water to give rise to these several states, can possess no weight. If
then heat is a material body, it must be devoid of weight—and hence, in
former times, heat was called an =imponderable= substance. It was
thought to be a kind of fluid, called =caloric=, which had no weight,
and which drove the particles of bodies asunder, when it entered them as
they were heated, and let them come together as it left and they grew
cool.
45. =The Phenomena of Heat are the Effects of a rapid Motion of the
Particles of Matter.=
This much, however, is certain: that heat can be caused by motion. Every
boy knows that a metal button may be made quite hot by rubbing it. A
skilful smith will hammer a piece of iron red hot. The axles of wheels
become red hot by rubbing against their bearings, if they are not
properly lubricated; and even two pieces of ice may be melted by the
heat evolved when they are rubbed together. And there are abundant other
reasons, as you will find when you study physics, for the belief that
the sensation we call heat, and all the phenomena which we ascribe to
heat, are the effects of the rapid motion of matter.
However, a quiescent body may be made hot without exhibiting the least
appearance of motion. The surface of the water in a tumbler at 100° is
just as unruffled as that of the same water at 32°. What, then, is meant
by saying that heat is a kind of motion, and that the greater the heat
in any body the greater the amount of motion in that body?
The answer to this question is that the motion which causes the
phenomena of heat, is not a visible motion of the whole mass of the hot
body, but a motion of the individual =particles= of which it is
composed. And each particle moves, not straight forward, but backwards
and forwards in the same space, so that its motion may be roughly
compared to that of a pendulum, or to that of the balance-wheel of a
watch. It is in fact a sort of =vibratory= movement; each vibration
taking place through a very short distance and with extreme rapidity.
The sensation of heat is caused by the vibratory movements of the
particles of matter, just as sound is so caused. The prongs of a
tuning-fork which has been struck, certainly vibrate, for you can see
them do so if the note is low. If you now put your ear at one end of a
long piece of timber and the handle of the vibrating tuning-fork is
placed upon the other end, the vibratory motion of the tuning-fork will
be communicated to the particles of the wood and will be loudly heard.
All the time the sound is heard the particles of the wood are vibrating.
Nevertheless, the wood as a whole does not move, but its particles swing
backwards and forwards through such a minute space that their motion is
imperceptible.
But what are these =particles= of matter which by their vibration give
rise to the phenomena of heat?