red hot. But what is caloric? Is it a thing,
or not a thing?

Of the various notions as to the cause of heat,
two only survive the sittings of time and
experiment the theory of emission, and the theory of
undulation. According to the first, the cause
of heat is a material, imponderable fluid, capable
of passing from one body to another, and whose
molecules are in a state of continual repulsion.
This fluid exists in all bodies in combination
with their ultimate atoms, whose actual contact
it prevents. In this case, caloric, a fluid, is a
thing. But material caloric is growing old, nay,
verging towards decrepitude. The word
remains, still hanging, like an autumnal leaf, on
the branch of science; the special substance,
i.e. the fluid of heat, is ebbing fast away. The
presence of caloric cannot be detected by its weight;
no human instrument sufficiently delicate for that
purpose has yet been invented. But, say some
philosophers, we must not conclude, from our
inability to weigh it, that it has no weight.

The undulatory theory holds that heat is produced
by a vibratory movement of the molecules
or atoms of hot bodies, which movement is
transmitted to the molecules of other bodies
through the medium of an extraordinarily subtle
and elastic fluid, called ether, in which it is
propagated in the same way as waves of sound
are in the air. The hottest bodies, then, are
those whose vibrations have the greatest breadth
and the greatest rapidity; and the intensity of
heat is no other than the resultant of the
vibrations of the molecules. On the first
hypothesis, the molecules of cooling bodies lose their
caloric; on the second, they only lose their
motion. On this latter supposition, Fire utterly
ceases to be a thing (just as sound is not a
thing), but a pulsation of aerial waves. Sound is
motion; and although in the earlier periods of
philosophy the identity of sound and motion was
not traced out, we now so readily resolve sound
into motion, that to those familiar with acoustics
its phenomena immediately present to the mind
the idea of motion of ordinary matter. And as,
in common parlance, we speak of sound moving
although sound, is motion, it requires no great
stretch of imagination to conceive heat, light, and
electricity as motions, and not as things moving.

With undulatory philosophers, then, Fire is an
action, a change, a motion, but it is no thing; it
consists of no body, substance, or material fluid,
ponderable or imponderable. The only things in the
case are the ether, whose vibrations constitute heat,
amounting to Fire, when their intensity is
sufficiently great, and the things acted upon, altered,
consumed, or burnt by the said heat or fire.

In strictness, therefore, the word Fire can
only be employed as a general expression
comprising a multitude of pnenomena heat, dilatation,
fusion, evaporation, &c. which are due to
what is called caloric, a fluid according to some,
a modification or molecular motion of matter
according to others. Fire, popularly so called,
is the result of combustion—of burning a
something which is its fuel. But combustion
comprises every phenomenon in which any body
whatever combines either with the oxygen in the
air, or with pure oxygen in a closed vessel
artificially prepared. All bodies capable of such
combustion have a right to be called combustibles.

It is not necessary that a body should
become actually inflamed on exposure to the air,
to be ranked amongst combustibles; it suffices
that it should have the acknowledged property
of absorbing more or less rapidly the oxygen
gas contained in atmospheric air, or of vitiating
it, and rendering it incapable of maintaining
sensible combustion. Often even the most inflammable
matters known, burn slowly and completely
exhaust their combustible properties
without producing any flame or other sensible
phenomenon by whose presence men usually
recognise combustibility; and after such slow
combustion, without flame or ardent heat, they
are not the less for that burnt bodies.

Slow combustion is constantly going on in
every warm-blooded animal. The result of
starvation, of deficient fuel, is feebler and feebler
animal heat, until the fuel is all burnt out and
deadly cold ensues. In animals, too, the
hydrogen and carbon in the food they consume is
the source of power; the horse without hay
and oats is as powerless as the steam-engine
without coals, or the voltaic battery without zinc.
In the production, however, of mechanical power
by heat, Nature far distances art in its present
state. According to some careful estimates, the
most economical of our furnaces consume from
ten to twenty times as much fuel to produce the
same quantity of heat as an animal produces;
and Mattucci found that, from a given consumption
of zinc in a voltaic battery, a far greater
mechanical effect could be produced by making
it act on the limbs of a recently killed frog
(notwithstanding the manifold defects of such an
arrangement, and its inferiority to the action of
the living animal), than when the same battery
was made to produce mechanical power, by acting
on an electro-magnetic or other artificial
motor apparatus. The ratio in his experiments,
was nearly six to one.

Outward changes in the forms of matter
betray that combustion is going on. The popular
proverb truly says that there is no smoke without
Fire. But there are a multitude of chemical
facts in which much heat is disengaged, and
even much light is set at liberty, without any
real combustion taking place. All combustion
supposes a chemical affinity, a relative attraction,
between the burning body and the oxygen,
which serves to aid the burning.

It is a grand step to have disabused our minds
of the notion that heat or Fire is A THING,
because we are then immediately led to the
suspicion that it may be A FORCE. We are accustomed
to think of only one kind of force, the
force of gravity, the most obvious and the most
constantly felt of all the forces of nature; but
that others exist must be evident to every one
who has performed, or even witnessed, a
chemical or electrical experiment.

The physical forces now acknowledged are
Heat, Light, Electricity, Magnetism, Chemical