Although the idea of a primal “element” or common principle is to be found in every old-world philosophical system, the ancient philosophers were by no means in agreement as to its character. Anaximenes, who lived _circa_ 500 B.C., taught that it was air, Herakleitos of Ephesus that it was fire, and Pherekides that it was earth. The supposition that a single primordial principle could be made to account for all forms of matter and all the phenomena and manifestations of the material world had its difficulties. Attempts to group qualities as principles, and to construct from these principles the universe, were indeed made even prior to the age of Thales. It was a comparatively simple evolutionary step to regard these principles or “elements” as mutually convertible. Anaximenes’ theory of the formation of rain was an implicit admission of such convertibility. This philosopher taught that rain came by the condensation of clouds, which in their turn were formed by the condensation of air. Everything comes from air and everything returns to air. That water might be converted by fire into air was surmised from the earliest times. Such a supposition naturally sprung from the circumstance that water was everywhere recognised to disappear or to pass into the air under the influence of fire or solar heat. The supposition had grown into a fixed belief in the Middle Ages. Even Priestley, as late as the end of the eighteenth century, imagined for a time that he had obtained proof of such a mutual conversion. The possibility of the transmutation of water into earth was a belief current through twenty centuries, and was only definitely and finally disproved by Lavoisier in 1770. The conception of fire as the primal principle has its germ in the fire- or sun-worship of the Chaldeans, Scythians, Persians, Parsees, and Hindus, and it is not difficult to trace, therefore, how heat came to be regarded either as antecedent to, or as associated with, the other primal principles. Empedokles, apparently, was the first whose name has come down to us to reintroduce the definite conception of four primal elements—fire, air, water, and earth. These he regarded as distinct, and incapable of being transmuted, but as forming all varieties of matter by intermixture in various proportions. These principles he deified, Zeus being the personification of the element of fire, Here of air, Nestis of water, and Aidoneous of earth.