It has been contended by some that the medium which conveys the
impression of light through transparent, bodies, is necessarily more
dense within the body than without; but according to this theory the
converse is true. A ray of light is a mechanical impulse, propagated
through an elastic medium, and, like a wave in water, tends to the side
of least resistance. Within a refracting body the ether is rarefied, not
only by the proximity of the atoms of the body (or its density), but
also by the motions of those atoms; so that if two _simple_ gases of
different specific gravity be made equal in density by compression,
their refraction will be approximately as their specific heats. In the
case of solids and liquids, or even compound gases, there is a continual
absorption of motion to produce the cohesion of composition and
aggregation. And the specific heats of compound gases will be found
greater than those of simple gases, in proportion to the loss of volume
by combination, _ceteris paribus_. If impenetrability be a law of
matter, the more a portion of atomic matter is condensed, the less ether
will be found in the same space. The same is also true when the natural
density or specific gravity of a gas is greater than that of another.
And the lighter the gas, the more will this circumstance vitiate the
experiments to determine its specific heat. There is, therefore, this
great source of fallacy in such experiments, viz.: that the ether
permeates all fluids and solids, and that _its specific heat probably
far exceeds that of all other matter_. This is a fundamental position of
the theory, in support of which we will introduce a fact announced by
M. V. Regnault, which was published in the Comptes Rendus of the French
Academy for April, 1853. He says: "In the course of my researches I have
encountered, indeed, at every step, anomalies which appeared to me
inexplicable, in accordance with the theories formally recognized. For
the sake of illustration I will quote one instance: 1st, a mass of gas,
under a pressure of ten atmospheres, is contained in a space which is
suddenly doubled; the pressure falls to five atmospheres. 2d. Two
reservoirs of equal capacity are placed in a calorimeter; the one is
filled with a gas, under a pressure of ten atmospheres; the second is
perfectly empty. In these two experiments, the initial and final
conditions of the gas are the same; but this identity of condition is
accompanied by calorific results which are very different; for while in
the former experiment there is a reduction of temperature, in the second
the calorimeter does not indicate the slightest alteration of
temperature." This experiment tends to confirm the theory. In the first
experiment, the sudden doubling of the space causes the ether also to
expand, inasmuch as the sides of the vessel prevent the instantaneous
passage of the external ether. In the second, both vessels are full, one
of ether, and the other of air mixed with ether; so that there is no
actual expansion of the space, and consequently no derangement of the
quantity of motion in that space.