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.