The modern atomistic theory, which is regarded as an indispensable instrument in chemistry to-day, must be carefully distinguished from the old philosophic atomism which was taught more than two thousand years ago by a group of distinguished thinkers of antiquity--Leucippus, Democritus, and Epicurus: it was considerably developed and modified later on by Descartes, Hobbes, Leibnitz, and other famous philosophers. But it was not until 1808 that modern atomism assumed a definite and acceptable form, and was furnished with an empirical basis by Dalton, who formulated the "law of simple and multiple proportions" in the formation of chemical combinations. He first determined the atomic weight of the different elements, and thus created the solid and exact foundation on which more recent chemical theories are based; these are all _atomistic_, in the sense that they assume the elements to be made up of homogeneous, infinitesimal, distinct particles, which are incapable of further analysis. That does not touch the question of the real nature of the atoms--their form, size, psychology, etc. These atomic qualities are merely hypothetical; while the _chemistry_ of the atoms, their "chemical affinity"--that is, the constant proportion in which they combine with the atoms of other elements--is empirical.[25] The different relation of the various elements towards each other, which chemistry calls "affinity," is one of the most important properties of ponderable matter; it is manifested in the different relative quantities or proportions of their combination in the intensity of its consummation. Every shade of inclination, from complete indifference to the fiercest passion, is exemplified in the chemical relation of the various elements towards each other, just as we find in the psychology of man, and especially in the life of the sexes. Goethe, in his classical romance, _Affinities_, compared the relations of pairs of lovers with the phenomenon of the same name in the formation of chemical combinations. The irresistible passion that draws Edward to the sympathetic Ottilia, or Paris to Helen, and leaps over all bounds of reason and morality, is the same powerful "unconscious" attractive force which impels the living spermatozoon to force an entrance into the ovum in the fertilization of the egg of the animal or plant--the same impetuous movement which unites two atoms of hydrogen to one atom of oxygen for the formation of a molecule of water. This fundamental _unity of affinity in the whole of nature_, from the simplest chemical process to the most complicated love story, was recognized by the great Greek scientist, Empedocles, in the fifth century B.C., in his theory of "the love and hatred of the elements." It receives empirical confirmation from the interesting progress of cellular psychology, the great significance of which we have only learned to appreciate in the last thirty years. On those phenomena we