| Below is an excerpt from James P. Joule, “On the Mechanical
Equivalent of Heat,” Philosophical Transactions of the Royal
Society, 140:61-82 (1850). For a long time it had been a favorite hypothesis that
heat consists of “a force or power belonging to bodies,” but it
was reserved for Count Rumford to make the first experiments
decidedly in favor of that view. That justly celebrated natural
philosopher demonstrated by his ingenious experiments that the
very great quantity of heat excited by the boring of cannon could
not be ascribed to a change taking place in the calorific capacity
of the metal; and he therefore concluded that the motion of the
borer was communicated to the particles of metal, thus producing
the phenomena of heat: "It appears to me," he remarks, "extremely
difficult, if not quite impossible, to form any distinct idea of
anything, capable of being excited and communicated, in the manner
the heat was excited and communicated in these experiments, except
it be motion." One of the most important parts of Count Rumford's
paper, though one to which little attention has hitherto been
paid, is that in which he makes an estimate of the quantity of
mechanical force required to produce a certain amount of heat.
Referring to his third experiment, he remarks that the "total
quantity of ice-cold water which, with the heat actually generated
by friction, and accumulated in 2h 30m, might have been heated
180°, or made to boil, = 26.58 lbs." In the next page he states
that "the machinery used in the experiment could easily be carried
round by the force of one horse (though, to render the work
lighter, two horses were actually employed in doing it)." Now the
power of a horse is estimated by Watt at 33,000 foot-pounds per
minute, and therefore if continued for two hours and a half will
amount to 4,950,000 foot-pounds which, according to Count
Rumford's experiment, will be equivalent to 28.58 lbs of water
raised 180°. Hence the heat required to raise a lb of water 1°
will be equivalent to the force [energy] represented by 1034
foot-pounds. This result is not very widely different from that
which I have deduced from my own experiments related in this
paper, viz., 772 foot-pounds; and it must be observed that the
excess of Count Rumford’s equivalent is just such as might have
been anticipated from the circumstance, which he himself mentions,
that “no estimate was made of the heat accumulated in the wooden
box, nor of that dispersed during the experiments.” About the end of the last century Sir Humphry Davy
communicated a paper to Dr. Beddoes’ West Country Contributions,
entitled, “Researches on Heat, Light, and Respiration,” in which
he gave ample confirmation to the views of Count Rumford. By
rubbing two pieces of ice against one another in the vacuum of an
air pump, part of them was melted, although the temperature of the
receiver was kept below the freezing point. The experiment was
the more decisively in favor of the doctrine of the immateriality
of heat, inasmuch as the capacity of ice for heat is much less
than that of water. It was therefore with good reason that Davy
drew the inference that “the immediate cause of the phenomena of
heat is motion, and the laws of its communication are precisely
the same as the laws of the communication of motion.” The researches of Dulong on the specific heat of
elastic fluids were rewarded by the discovery of the remarkable
fact that “equal volumes of all the elastic fluids, taken at the
same temperature, and under the same pressure, being compressed or
dilated suddenly to the same fraction of their volume, disengage
or absorb the same absolute quantity of heat.” This law is of the
utmost importance in the development of the theory of heat,
inasmuch as it proves that the calorific effect is, under certain
conditions, proportional to the force expended. In 1834 Dr. Faraday demonstrated the “Identity of the
Chemical and Electrical Forces.” This law, along with others
subsequently discovered by that great man, showing the relations
which subsist between magnetism, electricity, and light, have
enabled him to advance the idea that the so-called imponderable
bodies are merely the exponents of different forms of force.
Mr. Grove and M. Mayer have also given their powerful advocacy to
similar views. My own experiments in reference to the subject were
commenced in 1840, in which year I communicated to the Royal
Society my discovery of the law of the heat evolved by voltaic
electricity, a law from which the immediate deductions were drawn,
¾first, that the heat evolved by any voltaic pair is proportional,
caeteris paribus, to its intensity or electromotive force; and
second, that the heat evolved by the combustion of a body is
proportional to the intensity of its affinity for oxygen. I thus
succeeded in establishing relations between heat and chemical
affinity. In 1843 I showed that the heat evolved by
magneto-electricity is proportional to the force [energy]
absorbed; and that the force of the electromagnetic engine is
derived from the force of chemical affinity in the battery, a
force which otherwise would be evolved in the form of heat: from
these facts I considered myself justified in announcing “that the
quantity of heat capable of increasing the temperature of a lb of
water by one degree of Fahrenheit’s scale, is equal to, and may be
converted into, a mechanical force capable of raising 838 lbs to
the perpendicular height of one foot.” In a subsequent paper, read before the Royal Society
in 1844, I endeavored to show that the heat absorbed and evolved
by the rarefaction and condensation of air is proportional to the
force [energy] evolved and absorbed in those operations. The
quantitative relation between force [work] and heat deduced form
these experiments, is almost identical with that derived from the
electromagnetic experiments just referred to, and is confirmed by
the experiments of M. Séguin on the dilatation of steam. From the explanation given by Count Rumford of the
heat arising from the friction of solids, one might have
anticipated, as a matter of course, that the evolution of heat
would also be detected in the friction of liquid and gaseous
bodies. Moreover there were many facts, such as, for instance,
the warmth of the sea after a few days of stormy weather, which
had long been commonly attributed to fluid friction. Nevertheless
the scientific world, preoccupied with the hypothesis that heat is
a substance, and following the deductions drawn by Pictet from
experiments not sufficiently delicate, have almost unanimously
denied the possibility of generating heat in that way. The first
mention, so far as I am aware, of experiments in which the
evolution of heat from fluid friction is asserted, was in 1842 by
M. Mayer, who states that he has raised the temperature of water
from 12°C to 13°C, by agitating it, without however indicating the
quantity of force [energy] employed, or the precautions taken to
secure a correct result. In 1843 I announced the fact that “heat
is evolved by the passage of water through narrow tubes,” and that
each degree of heat per lb of water required for its evolution in
this way a mechanical force represented by 770 foot-pounds.
Subsequently in 1845, and 1847, I employed a paddle wheel to
produce the fluid friction, and obtained the equivalents 781.5,
782.1 and 787.6, respectively, from the agitation of water, sperm
oil, and mercury. Results so closely coinciding with one another,
and with those previously derived from experiments with elastic
fluids and the electromagnetic machine, left no doubt on my mind
as to the existence of an equivalent relations between force
[energy] and heat; but still it appeared of the highest importance
to obtain that relation with still greater accuracy. This I have
attempted in the present paper. |
