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II

Our daily lives go on for the most part in two worlds, the world of mechanical transposition and the world of chemical transformations, but we are usually conscious only of the former. This is the visible, palpable world of motion and change that rushes and roars around us in the winds, the storms, the floods, the moving and falling bodies, and the whole panorama of our material civilization; the latter is the world of silent, invisible, unsleeping, and all-potent chemical reactions that take place all about us and is confined to the atoms and molecules of matter, as the former is confined to its visible aggregates.

Mechanical forces and chemical affinities rule our physical lives, and indirectly our psychic lives as well. When we come into the world and draw our first breath, mechanics and chemistry start us on our career. Breathing is a mechanical, or a mechanico-vital, act; the mechanical principle involved is the same as that involved in the working of a bellows, but the oxidation of the blood when the air enters the lungs is a chemical act, or a chemico-vital act. The air gives up a part of its oxygen, which goes into the arterial circulation, and its place is taken by carbonic-acid gas and watery vapor. The oxygen feeds and keeps going the flame of life, as literally 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 of hydrogen, unite and are lost in each other; it is a marriage wherein the two or three become one. In dealing with the molecules and atoms of matter we are in a world wherein the laws of solid bodies do not apply; friction is abolished, elasticity is perfect, and place and form play no part. We have escaped from matter as we know it, the solid, fluid, or gaseous forms, and are dealing with it in its fourth or ethereal estate. In breathing, the oxygen goes into the blood, not to stay there, but to unite with and bring away the waste of the system in the shape of carbon, and re-enter the air again as one of the elements of carbonic-acid gas, CO_{2}. Then the reverse process takes place in the vegetable world, the leaves breathe this poisonous gas, release the oxygen under the chemistry of the sun’s rays, and appropriate and store up the carbon. Thus do the animal and vegetable worlds play into each other’s hands. The animal is dependent upon the vegetable for its carbon, which it releases again, through the life processes, as carbonic-acid gas, to be again drawn into the cycle of vegetable life.