irth to the whole vegetable
world, with all its varied charms of colour, form, and property, and, as
we have seen in the last chapter, on them animal life itself depends.

[Sidenote: Of heat; reflexion; refraction.] The conclusions arrived at
in optics necessarily entered as fundamental ideas in thermotics, or the
science of heat; for radiant heat moves also in straight lines,
undergoes reflexion, refraction, double refraction, polarization, and
hence the theory of transverse vibrations applies to it. Heat is
invisible light, as light is visible heat. Correct notions of radiation
originated with the Florentine academicians, who used concave mirrors;
and, in the cold-ray experiment, masses of ice of five hundred pounds
weight. The refraction of invisible heat was ascertained in consequence
of the invention of the thermoelectric pile. Its polarization and
depolarization soon followed. Already had been demonstrated the
influence of the physical state of radiant surfaces, and that the heat
comes also from a little depth beneath them. [Sidenote: Exchanges of
heat.] The felicitous doctrine of exchanges of heat imparted true ideas
of the nature of calorific equilibrium and the heating and cooling of
bodies, and offered an explanation of many phenomena, as, for instance,
the formation of dew. [Sidenote: The dew, nature of.] This deposit of
moisture occurs after sunset, the more copiously the clearer the sky; it
never appears on a cloudy night; it neither ascends from the ground like
an exhalation, nor descends like a rain. It shows preferences in its
manner of settling, being found on some objects before it is on others.
All these singular peculiarities were satisfactorily explained, and
another of the mysteries, the unaccountable wonders of the Middle Ages,
brought into the attitude of a simple physical fact.

[Sidenote: Incandescence. Physical instruments.] It is impossible, in a
limited space, to relate satisfactorily what has been done respecting
ignition, the production of light by incandescence, the accurate
measurement of the conductibility of bodies, the determination of the
expansions of solids, liquids, gases, under increasing temperature, the
variations of the same substance at different degrees, the heat of
fluidity and elasticity, and specific heat, or to do justice to the
great improvements made in all kinds of instruments--balances,
thermometers, contrivances for linear and angular measures, telescopes,
microsco