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.
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