philosopher made great strides in his studies of the physical properties
of matter and the application of these properties in mechanics, as the
steam-engine, the balloon, the optic telegraph, the spinning-jenny, the
cotton-gin, the chronometer, the perfected compass, the Leyden jar, the
lightning-rod, and a host of minor inventions testify. In a speculative
way he had thought out more or less tenable conceptions as to the ultimate
nature of matter, as witness the theories of Leibnitz and Boscovich and
Davy, to which we may recur. But he had not as yet conceived the notion
of a distinction between matter and energy, which is so fundamental to
the physics of a later epoch. He did not speak of heat, light, electricity,
as forms of energy or "force"; he conceived them as subtile forms of matter
- as highly attenuated yet tangible fluids, subject to gravitation and
chemical attraction; though he had learned to measure none of them but
heat with accuracy, and this one he could test only within narrow limits
until late in the century, when Josiah Wedgwood, the famous potter, taught
him to gauge the highest temperatures with the clay pyrometer.
He spoke of the matter of heat as being
the most universally distributed fluid in nature; as entering in some degree
into the composition of nearly all other substances; as being sometimes
liquid, sometimes condensed or solid, and as having weight that could be
detected with the balance. Following Newton, he spoke of light as a "corpuscular
emanation" or fluid, composed of shining particles which possibly are transmutable
into particles of heat, and which enter into chemical combination with
the particles of other forms of matter. Electricity he considered a still
more subtile kind of matter-perhaps an attenuated form of light. Magnetism,
"vital fluid," and by some even a "gravic fluid," and a fluid of sound
were placed in the same scale; and, taken together, all these supposed
subtile forms of matter were classed as "imponderables."
This view of the nature of the "imponderables"
was in some measure a retrogression, for many seventeenth- century philosophers,
notably Hooke and Huygens and Boyle, had held more correct views; but the
materialistic conception accorded so well with the eighteenth- century
tendencies of thought that only here and there a philosopher like Euler
called it in question, until well on towards the close of the century.
Current speech referred to the materiality of the "imponderables " unquestioningly.
Students of meteorology - a science that was just dawning - explained atmospheric
phenomena on the supposition that heat, the heaviest imponderable, predominated
in the lower atmosphere, and that light, electricity, and magnetism prevailed
in successively higher strata. And Lavoisier, the most philosophical chemist
of the century, retained heat and light on a par with oxygen, hydrogen,
iron, and the rest, in his list of elementary substances.