In any attempt to study the evolution of matter it is necessary to begin with its simplest known forms, the so-called chemical elements. During a great part of the nineteenth century many philosophical chemists held a vague belief that these elements were not distinct entities but manifestations of one primal substance-the protyle, as it is sometimes called. Other chemists, more conservative, looked askance at all such speculations and held fast to what they regarded as established facts. To them an element was something distinct from other kinds of matter, a substance which could neither be decomposed nor transmuted into anything else. This belief, however, was based entirely upon negative evidence-the inadequacy of our existing resources to produce such sweeping changes. Many important facts were ignored, and especially the fact that the elements are connected by very intimate relations, such as are best shown in the periodic law of Mendeleef, who, from gaps in his table of atomic weights, predicted the existence of three unknown metals, which have since been discovered. For these metals, scandium, gallium, and germanium, he foretold not only their atomic weights but also their most characteristic physical properties and the sort of compounds that each one would form. His prophecies have been verified in every essential particular. One obvious conclusion was soon drawn from Mendeleef's "law," although he was too cautious to admit it, namely, that the chemical elements must have had some community of origin. The philosophical speculations as to their nature were fully justified.