Although more than one philosopher of the seventeenth and eighteenth
centuries suggested the identity of heat and molecular motion, the
impression made was not lasting, and up to very near the beginning
of the nineteenth century the _caloric_ theory was accepted almost
without dispute. This theory implied that heat was a subtle fluid,
definite quantities of which were added to or subtracted from material
substances when they became hot or cold. As carefully conducted
experiments seemed to show that a body weighed no more or no less when
hot than when cold, it was necessary to attribute to this fluid called
caloric the mysterious property of imponderability, that is, unlike all
forms of ordinary matter, it possessed no weight. To avoid calling it
matter, it was by many classed with light, electricity, and magnetism,
as one of the _imponderable agents_. Various other properties were
attributed to caloric, necessary to the reasonable explanation of a
steadily increasing array of experimental facts. It was declared to be
elastic, its particles being mutually self-repellent. It was thought
to attract ordinary matter, and an ingenious theory of caloric was
constructed, modelled upon Newton’s famous but erroneous corpuscular
theory of light. During the latter part of the eighteenth century
Joseph Black, professor in the Universities of Glasgow and Edinburgh,
developed his theory of _latent heat_, which, although founded upon a
false notion of the nature of heat, was a most important contribution
to science. The downfall of the caloric theory must be largely credited
to the work of a famous American who published the results of his
experiments just at the close of the eighteenth century. Benjamin
Thompson, generally known as Count Rumford, was born in the town of
Woburn, Massachusetts, in 1753. His inclination towards physical
experimentation was strong in his early youth, and he received much
instruction and inspiration from the lectures of Professor John
Winthrop, of Harvard College, some of which he was enabled to attend
under trying conditions. Having received special official consideration
by appointment to office under one of the colonial governors, he was
accused at the breaking out of the Revolutionary War of a leaning
towards Toryism, and was thus prevented from making his career among
his own people. At the age of twenty-two years he fled to England,
returning to America only for a brief period in command of a British
regiment. In England he soon became eminent as an experimental
philosopher, and in 1778 became a Fellow of the Royal Society. He
afterwards entered the service of the Elector of Bavaria, by whom
he was made a Count of the Holy Roman Empire. In 1799 he returned
to London and founded the “Royal Institution,” which was destined
during the next hundred years to surpass all other foundations in the
richness and importance of its contributions to physical science. It
was while at Munich that Rumford made his famous experiments on the
nature of heat, to which he had been led by observing the great amount
of heat generated in the boring of cannon. Finding that he was able
to make a considerable quantity of water actually boil by the heat
generated by a blunt boring tool, he concluded that the supply of heat
from such a source was practically inexhaustible and that it could be
generated continuously if only the motion of the tool under friction
was kept up. He declared that anything which could thus be produced
without limitation by an insulated body or system of bodies could not
possibly be a material substance, and that under the circumstances of
the experiment, the only thing that was or could be thus continuously
communicated was _motion_.
Count Rumford’s conclusions were not for a long time accepted. Davy,
the brilliant professor and eloquent lecturer at the newly established
Royal Institution, espoused the mechanical theory of heat and made
the striking experiment of melting two pieces of ice by rubbing them
together remote from any source of heat. His contemporary, Thomas
Young, who overturned Newton’s corpuscular theory of light and showed
that it was a wave phenomenon, also advocated Rumford’s notion of the
nature of heat, but even among physicists of high rank it had made
little headway as late as the middle of the nineteenth century. In
the eighth edition of the Encyclopædia Britannica, published in 1856,
the immediate predecessor of the current issue, heat is defined as “a
material agent of a peculiar nature, highly attenuated.” And this, in
spite of the fact that previous to that date the mechanical theory had
been completely proved by the labors of Mayer, Joule, Helmholtz, and
William Thomson (Lord Kelvin). By these men a solid foundation for the
theory had been found in a great physical law of such importance that
it is justly considered to be the most far-reaching generalization in
natural philosophy since the time of Newton. Some account of this law
and its discovery will be given later in this paper.
Among the most important of the century’s contributions to our
knowledge of heat must be included the work of Fourier, as embodied
in his _Theorie Analytique de la Chaleur_, published in 1822. Joseph
Fourier was born in 1768, and died in 1830. He belonged to that
splendid group of philosophers of which the French nation may always
be proud, whose work constitutes a large part of the lustre of
intellectual France during her most brilliant period, the later years
of the eighteenth and the earlier years of the nineteenth century. His
contemporaries included such men as Laplace, Arago, Lagrange, Fresnel,
and Carnot. Fourier wrote especially of the movement of heat in solids,
and as his thesis depended in no way on the nature of heat it will
always be regarded as a classic. His assumption that conductivity was
independent of temperature was shortly proved to be erroneous, but
his general argument and conclusions were not greatly affected by
this discovery. His work is one of the most beautiful examples yet
produced of the application of mathematics to physical research, and
mathematical and physical science were equally enriched by it. In
its broader aspects his law of conduction includes the transfer of
electricity in good conductors, and is the real basis of Ohm’s law.
One of the most skillful and successful experimenters in heat was also
a Frenchman, Henri Victor Regnault (1810–78). He greatly improved the
construction and use of the thermometer, and was the first to discover
that the indications of an air thermometer and one of mercury did not
exactly agree, because they did not expand in the same degree for
equal increases of temperature. His most important work was on the
expansion of gases, vapor pressure, specific heat of water, etc., and
for careful, patient measuring he had a positive genius. Until he
proved the contrary it had been assumed that all gases had the same
coefficient of expansion, and Boyle’s law that the volume of a gas was
inversely proportional to its pressure had not been questioned. His
tables of the elastic force of steam have been of immense practical
value, but his studies of the expansion of gases are of greater
interest because they have pointed the way to one of the most important
accomplishments of the century, the liquefaction of all known gases.
During the earlier years of this century it was the custom to consider
vapors and gases as quite distinct forms of matter. Vapors always
came, by evaporation, from liquids, and could always be “condensed” or
reduced to the liquid form without difficulty, but it was not thought
possible to liquefy the so-called “permanent” gases. The first man to
attack the problem systematically was Michael Faraday, who, before
the end of the first third of the century, had liquefied several
gases, mostly by producing them by chemical reactions under pressure.
Several of the more easily reducible gases or vapors, such as ammonia,
sulphurous acid, and probably chlorine, had been previously liquefied
by cold, but a quarter of a century elapsed after Faraday’s researches
before the true relation of the liquid and gaseous states of matter
was understood, and it was found that both increase of pressure and
lowering of temperature were, in general, essential to the liquefaction
of a gas. It was Thomas Andrews, of Belfast, who first showed, in a
paper published in 1863, that there was a continuity in the liquid
and gaseous states of matter, that for each substance there was a
critical temperature at which it became a homogeneous fluid, neither a
liquid nor a gas: that above this temperature great pressure would not
liquefy, while below it the substance might exist as partly liquid and
partly gas. He pointed out the fact that for the so-called permanent
gases this critical temperature must be exceedingly low, and if such
temperature could be reached liquefaction would follow.
Subsequent progress in the liquefaction of gases came about by
following this suggestion. Very low temperatures were produced by
subjecting the gas to great reduction in volume by pressure, removing
the heat of compression by conduction and radiation, and then by sudden
expansion its temperature was greatly lowered. As early as 1877 two
Frenchmen, Pictet and Cailletet, had succeeded in liquefying oxygen,
hydrogen, nitrogen, and air. During the past twenty years great
improvements have been made in the methods of accomplishing these
transformations, so that to-day it is easy to produce considerable
quantities of all of the principal gases in a liquid form, and by
carrying the reduction in temperature still further portions of the
liquid may be changed to the solid state. The most important work along
this line has been done by Wroblewski and Olszewski, of the University
of Cracow, and Professor Dewar, of the Royal Institution in London.
Temperatures as low as about two hundred and fifty degrees C. below
the freezing-point of water have been produced, the “absolute zero”
being only two hundred and seventy-three degrees C. below that point.
These experiments promise to throw much light on the nature of matter,
and they are especially interesting as revealing its extraordinary
properties at extremely low temperatures. Among the most curious and
suggestive is the fact that the electrical resistance of pure metals
diminishes at a rate which indicates that at the absolute zero it would
vanish, and these metals would become perfect conductors of electricity.
The dynamics of heat, or “thermo-dynamics,” was an important field of
research in the early part of the century, on account of its practical
application to the improvement of the steam-engine. The science was
created by Carnot, who, in spite of the fact that his views regarding
the nature of heat were erroneous, discovered some of the most
interesting relations among the quantities involved, and discussed
their applications to the heat engines with great skill. Subsequent
contributors to the theory and practice of thermo-dynamics were
Clausius, Rankine, Lord Kelvin, and Professor Tait.
The mechanical theory of heat naturally led up to what has already been
referred to as the most important generalization in physical science
since the time of Newton, the doctrine of