rally as it feeds and keeps going the fires in
our stoves and furnaces.

Hence our most constant and vital relation to the world without is a
chemical one. We can go without food for some days, but we can exist
without breathing only a few moments. Through these spongy lungs of ours
we lay hold upon the outward world in the most intimate and constant
way. Through them we are rooted to the air. The air is a mechanical
mixture of two very unlike gases--nitrogen and oxygen; one very inert,
the other very active. Nitrogen is like a cold-blooded, lethargic
person--it combines with other substances very reluctantly and with but
little energy. Oxygen is just its opposite in this respect: it gives
itself freely; it is "Hail, fellow; well met!" with most substances, and
it enters into co-partnership with them on such a large scale that it
forms nearly one half of the material of the earth's crust. This
invisible gas, this breath of air, through the magic of chemical
combination, forms nearly half the substance of the solid rocks. Deprive
it of its affinity for carbon, or substitute nitrogen or hydrogen in its
place, and the air would quickly suffocate us. That changing of the dark
venous blood in our lungs into the bright, red, arterial blood would
instantly cease. Fancy the sensation of inhaling an odorless,
non-poisonous atmosphere that would make one gasp for breath! We should
be quickly poisoned by the waste of our own bodies. All things that live
must have oxygen, and all things that burn must have oxygen. Oxygen does
not burn, but it supports combustion.

And herein is one of the mysteries of chemistry again. This support
which the oxygen gives is utterly unlike any support we are acquainted
with in the world of mechanical forces. Oxygen supports combustion by
combining chemically with carbon, and the evolution of heat and light is
the result. And this is another mystery--this chemical union which takes
place in the ultimate particles of matter and which is so radically
different from a mechanical mixture. In a chemical union the atoms are
not simply in juxtaposition; they are, so to speak, inside of one
another--each has swallowed another and lost its identity, an impossible
feat, surely, viewed in the light of our experiences with tangible
bodies. In the visible, mechanical world no two bodies can occupy the
same place at the same time, but apparently in chemistry they can and
do. An atom of oxygen and one of carbon, or