eem that one of the chief offices of the early forests was to purify
the atmosphere of its undue proportion of carbonic acid, by absorbing
the carbon into their own substance, and eventually depositing it as
coal in the soil.

Another very important agent in the process of purifying the atmosphere,
and adapting it to the maintenance of a higher organic life, is found in
the deposits of lime. My readers will excuse me, if I introduce here a
very elementary chemical fact to explain this statement. Limestone is
carbonate of calcium. Calcium is a metal, fusible as such, and, forming
a part of the melted masses within the earth, it was thrown out with the
eruptions of Plutonic rocks. Brought to the air, it would appropriate
a certain amount of oxygen, and by that process would become oxide of
calcium, in which condition it combines very readily with carbonic acid.
Thus it becomes carbonate of lime; and all lime deposits played an
important part in establishing the atmospheric proportions essential to
the existence of the warm-blooded animals.

Such facts remind us how far more comprehensive the results of science
will become when the different branches of scientific investigation are
pursued in connection with each other. When chemists have brought their
knowledge out of their special laboratories into the laboratory of the
world, where chemical combinations are and have been through all time
going on in such vast proportions,--when physicists study the laws
of moisture, of clouds and storms, in past periods as well as in the
present,--when, in short, geologists and zoologists are chemists and
physicists, and _vice versa_,--then we shall learn more of the changes
the world has undergone than is possible now that they are separately
studied.

It may be asked, how any clue can be found to phenomena so evanescent as
those of clouds and moisture. But do we not trace in the old deposits
the rainstorms of past times? The heavy drops of a passing shower, the
thick, crowded tread of a splashing rain, or the small pinpricks of a
close and fine one,--all the story, in short, of the rising vapors,
the gathering clouds, the storms and showers of ancient days, we find
recorded for us in the fossil rain-drops; and when we add to this the
possibility of analyzing the chemical elements which have been absorbed
into the soil, but which once made part of the atmosphere, it is not too
much to hope that we shall learn something hereafter of t