_Argument from Prout’s Law._—The bearing of the hypothesis of Prout in relation to the evolution of the elements is first considered by Mr. Crookes. If that hypothesis were demonstrated it would show that the accepted elements are not co-equal, but have been formed by a process of expansion or evolution. According to this hypothesis the atomic weights of the elements are multiples by a series of whole numbers of the atomic weight of hydrogen. It is true that accurate determinations of the atomic weights of different elements do not by any means harmonise with the values which Prout’s Law requires; nevertheless the agreement in so many cases is so close that one can scarcely regard the coincidence as accidental. The atomic weights have been recalculated with extreme care by Professor F. W. Clarke, of Cincinnati, and he says that “none of the seeming exceptions are inexplicable. In short, admitting half-multiples as legitimate, it is more probable that the few apparent exceptions are due to undetected constant errors than that the great number of close agreements should be merely accidental.” In reference to this suggestion of Professor Clarke, Mr. Crookes thinks that it places the matter upon an entirely new basis. For, suppose the unit atom to be not hydrogen, but some element of still lower atomic weight, say _helium_, an element supposed by many authorities to exist in the sun and other stellar bodies—an element whose spectrum consists of a single ray, and whose vapour possesses no absorbent power, which indicates a remarkable simplicity of molecular constitution. Granting that helium exists, all analogy points, says Mr. Crookes, to its atomic weight being below that of hydrogen; and here, then, we have the very element with atomic weight half that of hydrogen required by Professor Clarke as the basis of Prout’s Law. _Argument from the earth’s crust._—The probable compound nature of the chemical elements, Mr. Crookes thinks, is better shown by a consideration of certain peculiarities in their occurrence in the earth’s crust. “We do not,” he says, “find them evenly distributed throughout the globe. Nor are they associated in accordance with their specific gravities: the lighter elements placed on or near the surface, and the heavier ones following serially deeper and deeper. Neither can we trace any distinct relation between local climate and mineral distribution. And by no means can we say that elements are always or chiefly associated in nature in the order of their so-called chemical affinities: those which have a strong tendency to form with each other definite chemical combinations being found together, whilst those which have little or no such tendency exist apart. We certainly find calcium as carbonate and sulphate, sodium as chloride, silver and lead as sulphides; but why do we find certain groups of elements, with little affinity for each other, yet existing in juxtaposition or commixture?”