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.

The act of breathing well illustrates our mysterious relations to Nature—­the cunning way in which she plays the principal part in our lives without our knowledge.  How certain we are that we draw the air into our lungs—­that we seize hold of it in some way as if it were a continuous substance, and pull it into our bodies!  Are we not also certain that the pump sucks the water up through the pipe, and that we suck our iced drinks through a straw?  We are quite unconscious of the fact that the weight of the superincumbent air does it all, that breathing is only to a very limited extent a voluntary act.  It is controlled by muscular machinery, but that machinery would not act in a vacuum.  We contract the diaphragm, or the diaphragm contracts under stimuli received through the medulla oblongata from those parts of the body which constantly demand oxygen, and a vacuum tends to form in the chest, which is constantly prevented by the air rushing in to fill it.  The expansive force of the air under its own weight causes the lungs to fill, just as it causes the bellows of the blacksmith to fill when he works the lever, and the water to rise in the pump when we force out the air by working the handle.  Another unconscious muscular effort under the influence of nerve stimulus, and the air is forced out of the lungs, charged with the bodily waste which it is the function to relieve.  But the wonder of it all is how slight a part our wills play in the process, and how our lives are kept going by a mechanical force from without, seconded or supplemented by chemical and vital forces from within.