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