The chief agent in causing the repulsion or separation of the particles
of bodies from each other is heat, or more correctly _caloric_, by
which is understood the unknown _cause_ of the effect called heat.
Philosophers are not agreed upon the nature of this wonderful agent. It
pervades all nature, is the cause of nearly all the changes that take
place both in organic and inorganic matter, and has great influence in
the meteorological phenomena which we observe in the atmosphere that
surrounds our planet. It appears to be intimately connected with light,
electricity, and magnetism--subjects which the genius of Faraday and
others have investigated, and by their discoveries brought us nearer to
the knowledge of the real nature of these most wonderful forces.
Caloric, then, exists in all bodies, and has a constant tendency to
equalize itself, as far at least as its outward manifestation, called
temperature, is concerned; for if a _hot_ body be brought near colder
ones, it will give up heat to them, until by its loss and their gain
they all become of the same temperature; and this proceeds more or
less rapidly, according as the original difference of temperature
was greater or less. Some other circumstances also influence this
equalization. The converse will take place on introducing a cold body
among warmer ones, when heat will be abstracted from all the bodies
within reach of its influence, until it has absorbed sufficient caloric
to bring its own temperature to an equality with theirs. This is the
true explanation of the apparent production of _cold_. When, for
instance, an iceberg comes across a ship’s course, it appears to _give
out_ cold, whereas it has abstracted the heat from the air and sea in
its neighborhood, and they in turn act upon the ship and everything in
it, until one common temperature is produced in all the neighboring
bodies.
It does not follow that the bodies thus equalized in temperature
contain equal quantities of caloric; far from it. Each body requires
a particular quantity of caloric to raise its temperature through a
certain number of degrees; and such quantity is called its _specific_
caloric. A pound of water, for instance, will take just twice as much
caloric as a pound of olive oil, to raise its temperature through the
same number of degrees; the _specific_ caloric of water is therefore
double that of oil. Mix any quantity of oil at 60 deg. of temperature
with an equal weight of water at 90 deg., and you will find the
temperature of the mixture to be nearly 80 deg., instead of only 74
deg. or 75 deg., showing that while the water has lost only 10 deg.
of caloric, the mixture has risen 20 deg. If the oil be at 90 deg.,
and the water at 60 deg., the resulting temperature will be only 70
deg., or thereabouts, instead of 75 deg., the mean; thus, here the hot
oil has lost 20 deg., while the mixture has risen only 10 deg.; the
water, then, contains at the same temperature _twice_ as much caloric
as the oil; its specific caloric is _double_ that of the oil. This
mean temperature does result when equal weights of the same body at
different temperatures are mixed together.
The sensations called heat and cold are by no means accurate measures
of the real temperature of any substances, for many causes influence
these sensations, some belonging to the substances themselves, others
to the state of our organs at the time. Every one has remarked that
metals in a warm room feel warmer, and in a cold room colder than
wooden articles, and these again than woolen or cotton articles of
dress or furniture; this arises from metals being what is termed better
_conductors_ of heat than wood, and this better than wool, &c., that
is, they give out or absorb caloric more rapidly than these last. Some
philosophers, wishing to ascertain how much heat the human body could
endure, had a room heated with stoves, every crevice being carefully
stopped, until the temperature rose so high that a beefsteak placed
on the table was sufficiently cooked to be eaten. They were dressed
in flannel, and could with impunity touch the carpets, curtains,
&c., in the room; but the iron handles, fire-irons, and all metallic
substances, burnt their fingers; and one who wore silver spectacles was
obliged to remove them to save his nose. The fallacy of our sensations
may be easily shown by taking two basins, placing in one some water at
100 deg., in another some water at as low a temperature as can easily
be procured--hold the right hand in one, the left in the other, for a
few minutes, and then mix them, and place both hands in the mixture; it
will feel quite _cold_ to the hand that had been in the hotter water,
and _hot_ to the other.
In order to arrive at a correct estimate of the temperature of bodies,
instruments are made use of called thermometers, or measurers of
heat, which show increase or diminution of temperature by the rising
or falling of a column of some fluid in a tube of glass, one end of
which is expanded into a bulb, and the other hermetically sealed.
This effect is produced by the expansion or swelling of the fluid as
caloric is added to, and its contraction when caloric is abstracted
from it. Colored spirits of wine, or quicksilver, are the most usual
thermometric fluids, and the tube containing them is fixed to a wooden
or metallic frame, on which certain divisions are marked, called
degrees.
That in general use in America is called Fahrenheit’s from the name
of the person who first introduced that particular scale. In this
thermometer, the point at which the mercury in the tube stands when
plunged into melting ice, is marked 32 degrees, and the distance
between that point and the point to which the mercury rises in boiling
water is divided into 180 equal parts, called degrees; so that water is
said to boil at 212 degrees = 180 degrees + 32 degrees. There are two
other scales of temperature used in different parts of the world, but
it is not worth while to notice them here.
Not only do different bodies at the same degree of temperature contain
very different quantities of caloric, but this also is the case with
the same body in different forms. Ice, water, and steam are three forms
of the same body, but ice at 32 degrees contains much less caloric
than water at the same temperature, and water at 212 degrees contains
much less caloric than steam (or water in a state of vapor) at that
temperature.
Place in a jar any given quantity of snow, or small pieces of ice, at
32 degrees, and in another the same weight of water at 32 degrees, pour
on each an equal weight of water at 172 degrees, and you will find
that in the first case the ice will be melted, but the temperature
will remain at 32 degrees, or thereabouts, while the temperature
of the water in the other vessel will have risen to 100 degrees or
thereabouts, being as near as possible the half of the excess of the
temperature of the hot water, 140 degrees over that of the cold, namely
70 degrees added to 32 degrees, the original temperature. Now, what
has become of the heat which was added to the ice, and is apparently
lost?--it is _absorbed_ by the ice in its passage to the fluid state;
so that water may be said to be a compound of ice and caloric.
Again, take 10 ounces of water at about 50 degrees, and add 1 oz. of
water at 212 degrees, and the temperature of the mixture will be about
66 degrees; then condense some steam at 212 degrees into another 10 oz.
of water until it has become 11 oz., and you will find the temperature
will be nearly 212 degrees. Why does the ounce of steam at 212 degrees
raise the temperature of the water so much higher than the ounce of
water at the same temperature? Obviously because it contains hidden in
its substance a vast quantity of caloric, not to be detected by the
thermometer; in fact, that steam is a compound of _water_ and caloric,
as water is a compound of _ice_ and caloric; and this caloric which
exists, more or less, in all bodies without producing any obvious
effect, is called _latent_ caloric, from the Latin verb _lateo_, to
lie hid. The quantity of caloric thus absorbed, as it were, by various
bodies, differs for each body, and for the same body in different
forms, as mentioned above.