f
questioned as to the chief properties of these constituents, he would
have replied, with equal facility, that these are among the most
important elements; that oxygen might almost be said to be the
life-giving principle, inasmuch as no air-breathing creature could get
along without it for many moments together; and that nitrogen is equally
important to the organism, though in a different way, inasmuch as it is
not taken up through the lungs. As to the water-vapor, that, of course,
is a compound of oxygen and hydrogen, and no one need be told of its
importance, as every one knows that water makes up the chief bulk of
protoplasm; carbonic-acid gas is also a compound of oxygen, the other
element this time being carbon, and it plays a quite different role in
the economy of the living organism, inasmuch as it is produced by the
breaking down of tissues, and must be constantly exhaled from the lungs
to prevent the poisoning of the organism by its accumulation; while
ammonia, which exists only in infinitesimal quantities in the air, is a
compound of nitrogen and hydrogen, introducing, therefore, no new
element.

If one studies somewhat attentively the relation which these elements
composing the atmosphere bear to the living organism he cannot fail to
be struck with it; and it would seem a safe inductive reasoning from the
stand-point of the evolutionist that the constituents of the atmosphere
have come to be all-essential to the living organism, precisely because
all their components are universally present. But, on the other hand,
if we consider the matter in the light of these researches regarding the
new gases, it becomes clear that perhaps the last word has not been said
on this subject; for here are four or five other elementary substances
which, if far less abundant than oxygen and nitrogen, are no less widely
distributed and universally present in the atmosphere, yet no one of
which apparently takes any chemical share whatever in ministering to the
needs of the living organism. This surely is an enigma.

Taking another point of view, let us try to imagine the real status of
these new gases of the air. We think of argon as connected with nitrogen
because in isolation experiments it remains after the oxygen has been
exhausted, but in point of fact there is no such connection between
argon and nitrogen in nature. The argon atom is just as closely in
contact with the oxygen in the atmosphere as with the nitrogen; it
sim