. Evaporation is not confined to water, but as our subject
has to deal with atmospheric phenomena we will speak of it only in its
relation to aqueous moisture.

The heat that is imparted to the earth's surface by the rays of the sun
is able to separate water into minute particles, which, when so
separated, form what is called vapor, which is transparent, as well as
much lighter than the air at the surface of the earth. Being lighter
than the air, it rises when disengaged and floats to the upper regions
of the atmosphere. The atmosphere will contain a certain amount of these
transparent globules of moisture in the spaces between its own
molecules. If the air is warm the molecules will be farther apart and it
will contain more moisture than when it is cold.

The process of evaporation is one of the most important in the catalogue
of nature's dynamics. Without it there would be no verdure on the hills,
no trees on the plains, no fields of waving grain, and no animal life
upon the land surface of the globe. Evaporation is nature's method of
irrigation, and the system is inaugurated on a grand scale, so that
there are but few neglected spots upon the face of the earth which
moisture, carried up from the great reservoirs of water, does not reach.
The rate of evaporation, other things being equal, depends upon the
extent of surface; therefore a smooth surface like that of the lake or
ocean will not send up as much vapor from a given area in square miles
as an equal area of land will do, when it is saturated with moisture,
for the reason that there is a much larger evaporating surface on a
square mile of land, owing to its inequalities, than upon an equal area
of smooth water. Of course, if the earth is dry there can be but little
evaporation. One of the effects of evaporation is to withdraw heat, and
so to produce cold in the substance from which the evaporation takes
place.

If we put water into a vial and drop regularly upon it some fluid that
evaporates readily it will extract the heat from the vial and the water
in it to such an extent that in a short time the water will be frozen.
In hot countries ice is manufactured on a large scale upon the principle
that we have just described. Water is put into shallow basins, excavated
in the earth, over which is placed some substance like straw that
readily radiates heat, and on the straw are placed porous bricks, that
are kept wet, thus furnishing a very large evaporating surface