. LAWS OF HEAT. WATER. 87

in this shape. We remark how suitable and
indispensable to the well-being of the creation it
is, that the fluid should possess the property of
assuming such a form under such circumstances.

The moisture which floats in the atmosphere is
of most essential use to vegetable life.* “ The
leaves of livi11g plants appear to act upon this
vapour in its elastic form, and to absorb it. Some
vegetables increase in weight from this cause
when suspended in the atmosphere and uncon-
nected with the soil, as the house-leek and the
aloe. In very intense heats, and when the soil
is dry, the life of plants seems to be preserved by
the absorbent power of their leaves.” It follows
from What has already been said, that, with an
increasing heat of the atmosphere, an increasing
quantity of vapour will rise into it, if supplied
from any quarter. Hence it appears that aqueous
vapour is most abundant in the atmosphere when
it is most needed for the purposes of life; and
that when other sources of moisture are cut ofl‘,
this is most copious.

4. Clouds are produced by aqueous vapour
when it returns to the state of water. This process
is condensation, the reverse of evaporation. \Vhen
vapour exists in the atmosphere, if in any manner
the temperature becomes lower than the con-
stituerzt tenzperatzzre, requisite for the maintei

* .011don. P219.