321. _Why does friction produce heat?_
Because all bodies contain _latent heat_, that is, heat that lies hid
in their substance, and the rubbings of two bodies against each other
_draws the latent heat to the excited surfaces_.
322. _Why does the rubbing of two surfaces together attract latent heat
to those surfaces?_
Because it is a law of nature that _heat_ shall always attend _motion_;
and it is generally found that the _intensity of heat_ bears a specific
relation to the _velocity of motion_.
323. _What are the sources of heat?_
The _rays_ of the _sun_, the _currents_ of _electricity_, the _action_
of _chemicals_, and the _motion_ of _substances_.
324. _Why does water freeze?_
Because its latent heat is partly _drawn off_ by the surrounding air.
325. _Why does ice melt?_
Because the heat, once latent in the water, but drawn off by the air,
_has returned_ to it, and restored the water to its former condition.
[Verse: "So teach us to number our days, that we may apply our
hearts unto wisdom." PSALM XC.]
326. _Why does water become steam?_
Because a larger amount of heat has entered into it than can remain
latent in water. The water therefore expands and rises in the form of
vapour, or _water attenuated by heat_.
327. _How many degrees of heat are latent, or hidden, in the different
states of water?_
In thawing _ice_, 140 deg. of caloric become latent; and in converting
the water into steam, 1,000 deg. more of caloric are be taken up.
Therefore, _ice_ requires to take up 1,140 deg. of latent caloric
before it becomes steam.
328. _What is the most modern theory of heat?_
It is this--that caloric, which produces heat, is an extremely _subtile
fluid_, of so refined a nature that it possesses no weight, yet is
capable of diffusing itself among the particles of the most solid
bodies.
It is also believed that--all bodies are subject to the action of two
opposing forces: one, the _mutual attraction_ of their _particles_; the
other, the _repulsive force_ of _caloric_--and that bodies exist in the
_æriform_, _fluid_, or _solid state_, _according to the predominance of
either the one or the other of these opposing forces_.
329. _How do we measure the quantity of caloric in any substance?_
It is impossible to determine the amount of caloric which any body
contains. Our _sensations_ would obviously be deceptive, since, if we
dipped the right hand in snow, and held the left hand before the fire,
and then immersed both hands in cold water, the water would feel _warm_
to the _right hand_ and _cold_ to the _left hand_.
But, as _caloric_ uniformly expands substances that are under its
influence, one of the bodies most sensitive to _calorific_ effects
has been selected to be the _indicator_ of the amount of _caloric_.
This substance is _quicksilver_; and the scale of measurement, and
the apparatus for exhibiting the rise or fall of the quicksilver,
constitute the _thermometer_.
330. _If it is impossible to measure the amount of caloric in any
substance, how can it be said that ice absorbs_ 140 _deg. in becoming
water?_
Those figures simply record the amount of calorie indicated by the
_thermometer_. The instrument will show with sufficient accuracy
the _relative amount_ of caloric in various bodies, or in the same
bodies _under different circumstances_, but it can never determine the
_precise amount of caloric_ in any one body.
[Verse: "Great is the Lord, and greatly to be praised in the city of
our God, in the mountain of his holiness."--PSALM XLVIII.]
331. _Why, if a hot and a cold body were placed near to each other,
would the cold one become warmer, and the hot one cooler?_
Because _free caloric_ (that is, caloric that is not latent,) always
exhibits a tendency to establish an _equilibrium_. If twenty bodies, of
different temperatures, were placed in the same atmosphere, they would
_all soon arrive at the same temperature_. The caloric would leave the
bodies of those of the _highest_, and find its way to those of the
_lowest_ temperature.
332. _How does caloric travel?_
It travels in _parallel rays_ in all directions with a velocity
approximating to that of light; and it passes through various bodies
with a rapidity proportionate to their power of _conduction_.
333. _Why does melted metal run like a stream of fluid?_
Because _caloric_ has passed into its substance, and, repelling its
particles, has separated them to that degree which produces fluidity.
334. _How do we know that it is caloric passing into the substance of
the metal which produces this effect?_
Because, as soon as a bar of metal begins to be heated, it _expands_
and _lengthens_. It continues to do so, until the heat arrives at that
point which _causes the metal to melt_.
335. _Why does the iron of an ironing-box sometimes become too large
for the box to receive it?_
Because _caloric_ has passed into the substance of the iron, and
_repelled its particles_, by which it has become expanded.
336. _Why does the iron enter the box when it has become partially
cooled?_
Because a portion of the caloric has left the iron, the particles of
which have _drawn closer together_, and contracted the mass.
[Verse: "Cast thy burden upon the Lord, and he shall sustain thee;
he shall never suffer the righteous to be moved."--PSALM LV.]
This effect is frequently observed by females in domestic life,
who, when they are ironing, or using the Italian irons, find that
the heated metal has been too much expanded to enter the box or
tube. They find it necessary to wait until the cooling of the iron
has had the effect of reducing its dimensions. The expansion of
bodies by heat is one of the grandest and most important laws of
nature. We are indebted to it for some of the most beautiful, as
well as the most awful, phenomena. And science has gained some of
its mightiest conquests through its aid. Yet frequently, though
quite unthought of, in the hands of the humble laundress, will
be found a most striking illustration of this wonderful force of
caloric.
337. _Are there any instances in which the abstraction of latent heat
will reduce the hulk of bodies?_
Yes, there are several. But the most familiar one is that which is
exhibited by mixing a _pint_ of the _oil of vitriol_ with a _pint_ of
_water_. _A considerable amount of heat will be evolved_; and it will
be found that the two pints of fluid _will not afterwards fill a quart
measure_.
338. _Is there any latent heat in air?_
Yes: a considerable amount. In a pint measure of air, though in no way
evident to our perceptions, there lurks sufficient caloric to raise a
piece of metal several inches square to glowing redness.
339. _How do we know that caloric exists in the air?_
It has been positively demonstrated by the invention of a small
condensing syringe, by which, through the rapid compression of a small
volume of air, a spark is emitted which ignites a piece of prepared
tinder.
340. _What is the cause of the spark when a horse's shoe strikes
against a stone?_
The _latent heat_ of the iron or the stone is set free by the _violent
percussion_. The same effect takes place when _flint_ strikes against
_steel_, as in the old method of obtaining a light with the aid of the
tinder-box.
[Verse: "The waters are laid as with a stone, and the face of the
deep is frozen."--JOB XXXVIII.]
What an eloquent lecture might be delivered upon the old-fashioned
tinder-box, illustrated by the one experiment of "striking a
light." In that box lie, cold and motionless, the Flint and Steel,
rude in form and crude in substance. And yet, within the breast of
each, there lies a spark of that grand element which influences
every atom of the universe; a spark which could invoke the fierce
agents of destruction to wrap their blasting flames around a
stately forest, or a crowded city, and sweep it from the face of
the world; or which might kindle the genial blaze upon the homely
hearth, and shed a radiant glow upon a group of smiling faces; a
spark such as that which rises with the curling smoke from the
village blackmith's forge--or that which leaps with terrific wrath
from the troubled breast of a Vesuvius. And then the tinder--the
cotton--the carbon: What a tale might be told of the cotton-field
where it grew, of the black slave who plucked it, of the white
toiler who spun it into a garment, and of the village beauty who
wore it--until, faded and despised, it was cast amongst a heap of
old rags, and finally found its way to the tinder-box. Then the
Tinder might tell of its hopes; how, though now a blackened mass,
soiling everything that touched it, it would soon be wedded to
one of the great ministers of nature, and fly away on transparent
wings, until, resting upon some Alpine tree, it would make its
home among the green leaves, and for a while live in freshness
and beauty, looking down upon the peaceful vale. Then the Steel
might tell its story, how for centuries it lay in the deep caverns
of the earth, until man, with his unquiet spirit, dug down to
the dark depths and dragged it forth, saying, "No longer be at
peace." Then would come tales of the fiery furnace, what Fire had
done for Steel, and what Steel had done for Fire. And then the
Flint might tell of the time when the weather-bound mariners,
lighting their fires upon the Syrian shore, melted silicious
stones into gems of glass, and thus led the way to the discovery
of the transparent pane that gives a crystal inlet to the light
of our homes; of the mirror in whose face the lady contemplates
her charms; of the microscope and the telescope by which the
invisible are brought to sight, and the distant drawn near; of
the prism by which Newton analysed the rays of light; and of the
photographic camera in which the sun prints with his own rays the
pictures of his own adorning. And then both Flint and Steel might
relate their adventures in the battle-field, whither they had gone
together; and of fights they had seen in which man struck down his
fellow-man, and like a fiend had revelled in his brother's blood.
Thus, even from the cold hearts of flint and steel, man might
learn a lesson which should make him blush at the "glory of war;"
and the proud, who despise the teachings of small things, might
learn to appreciate the truths that are linked to the story of a
"tinder-box."