ere in temperate zones. But no one can
say what may not be possible in times to come, when the data now being
gathered all over the world shall at last be co-ordinated, classified,
and made the basis of broad inductions. Meteorology is pre-eminently a
science of the future.

VI. MODERN THEORIES OF HEAT AND LIGHT

THE eighteenth-century 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