at clear space the thing which makes the cloud must pass. What, then,
is this thing which at one moment is transparent and invisible, and at
the next moment visible as a dense opaque cloud?

It is the _steam_ or _vapour of water_ from the boiler. Within the
boiler this steam is transparent and invisible; but to keep it in this
invisible state a heat would be required as great as that within the
boiler. When the vapour mingles with the cold air above the hot funnel,
it ceases to be vapour. Every bit of steam shrinks, when chilled, to a
much more minute particle of water. The liquid particles thus produced
form a kind of _water-dust_ of exceeding fineness, which floats in the
air, and is called a _cloud_.

Watch the cloud-banner from the funnel of a running locomotive; you see
it growing gradually less dense. It finally melts away altogether; and
if you continue your observations, you will not fail to notice that the
speed of its disappearance depends upon the character of the day. In
humid weather the cloud hangs long and lazily in the air; in dry weather
it is rapidly licked up. What has become of it? It has been reconverted
into true invisible vapour.

The _drier_ the air, and the _hotter_ the air, the greater is the amount
of cloud which can be thus dissolved in it. When the cloud first forms,
its quantity is far greater than the air is able to maintain in an
invisible state. But, as the cloud mixes gradually with a larger mass of
air, it is more and more dissolved, and finally passes altogether from
the condition of a finely-divided liquid into that of transparent vapour
or gas.

Make the lid of a kettle air-tight, and permit the steam to issue from
the spout; a cloud is formed in all respects similar to that issuing
from the funnel of the locomotive. To produce the cloud, in the case of
the locomotive and the kettle, _heat_ is necessary. By heating the water
we first convert it into steam, and then by chilling the steam we
convert it into cloud. Is there any fire in Nature which produces the
clouds of our atmosphere? There is: the fire of the sun.

When the sunbeams fall upon the earth, they heat it, and also the water
which lies on its surface, whether it be in large bodies, such as seas
or rivers, or in the form of moisture. The water being thus warmed, a
part of it is given off in the form of aqueous vapour, just as invisible
vapour passes off from a boiler when the water in it is heated by fire.
This vapour