James Hutton (1726-1797)

42

                               THEORY of RAIN.

according to the known laws of condensation, water must be
separated from the menstruum, and become visible by reflecting
light. In like manner, water may be rendered an invisible
elastic fluid, by means of heat alone; and this fluid, in being
cooled, will be condensed into water, and appear visible. But
it is now to be shown, that, when breath or steam becomes vi-
sible, in mixing with the atmosphere, this effect is not produced
in consequence of the general principles of heat and cold: That,
for the explanation of this appearance, there is required the
knowledge of a particular law; and that the effects of heat or
cold, in relation to air and vapour, proceed not always in ratios
which are equally increasing or diminishing.

IN order to determine the actual ratio of the dissolving
power of air, in relation to water in different degrees of
heat, or the ratio in which the power of heat converts fluid
water into elastic steam, we must consider the several ratios in
which this operation may proceed; for if, among all the con-
ceivable ways of proceeding, there shall be but one with which
natural appearances shall correspond, it will then be reasonable
to conclude, that this corresponding ratio is the particular law of
nature, and that appearances of this kind are thus to be explained.

THE dissolving power of air, in relation to water, may be
supposed to diminish as heat is increased; but this would be in-
consistent with natural appearances in general. Such a suppo-
sition, therefore, would be now superfluous. This power might
also be conceived as not affected with the increase or diminu-
tion of the degree of heat; and this supposition is agreeable to
the solution of sea-salt in water: But, as it is certainly not the
case with air and vapour, neither is this supposition to be made.
The general rule of dissolving and evaporating bodies, is to in-
crease with heat. This is now to be admitted as the case with
water evaporating in air, or when, by means of heat alone, it
is converted into steam; and it is only the ratio or measure of this
operation which here is to be made the subject of consideration.

THERE