Тин foundations of every one of the great branches of Natural Philosophy have been probed beyond the merely phenomenal or molar stage of their description. They are now treated, in large part at least, in the light of molecular or even sub-atomic ideas. Upon every one of them has been cast the illumination of the two great speculative modern expansions of the older physical theory. And, in turn, it lies with them, by appeal to the decision of experiment to discriminate amongst the various forms which these speculations may assume, and to lead to the recognition of their underlying compulsions, and so to the elimination of what is arbitrary in their formulations.