ake rain, and propose to
adduce all necessary proof of my thesis. The nature of sound is fully
understood, and so are the conditions under which the aqueous vapor in
the atmosphere may be condensed. Let us see how the case stands.

A room of average size, at ordinary temperature and under usual
conditions, contains about a quart of water in the form of invisible
vapor. The whole atmosphere is impregnated with vapor in about the same
proportion. We must, however, distinguish between this invisible vapor
and the clouds or other visible masses to which the same term is often
applied. The distinction may be very clearly seen by watching the steam
coming from the spout of a boiling kettle. Immediately at the spout the
escaping steam is transparent and invisible; an inch or two away a
white cloud is formed, which we commonly call steam, and which is seen
belching out to a distance of one or more feet, and perhaps filling a
considerable space around the kettle; at a still greater distance this
cloud gradually disappears. Properly speaking, the visible cloud is not
vapor or steam at all, but minute particles or drops of water in a
liquid state. The transparent vapor at the mouth of the kettle is the
true vapor of water, which is condensed into liquid drops by cooling;
but after being diffused through the air these drops evaporate and
again become true vapor. Clouds, then, are not formed of true vapor,
but consist of impalpable particles of liquid water floating or
suspended in the air.

But we all know that clouds do not always fall as rain. In order that
rain may fall the impalpable particles of water which form the cloud
must collect into sensible drops large enough to fall to the earth. Two
steps are therefore necessary to the formation of rain: the transparent
aqueous vapor in the air must be condensed into clouds, and the
material of the clouds must agglomerate into raindrops.

No physical fact is better established than that, under the conditions
which prevail in the atmosphere, the aqueous vapor of the air cannot be
condensed into clouds except by cooling. It is true that in our
laboratories it can be condensed by compression. But, for reasons which
I need not explain, condensation by compression cannot take place in
the air. The cooling which results in the formation of clouds and rain
may come in two ways. Rains which last for several hours or days are
generally produced by the intermixture of currents of air of diff