classes of atoms, each differing in its very essence from the others. As the case stands to-day the chemist deals with seventy-odd substances, which he calls elements. Each one of these substances is, as he conceives it, made up of elementary atoms having a unique personality, each differing in quality from all the others. As far as experiment has thus far safely carried us, the atom of gold is a primordial element which remains an atom of gold and nothing else, no matter with what other atoms it is associated. So, too, of the atom of silver, or zinc, or sodium--in short, of each and every one of the seventy-odd elements. There are, indeed, as we shall see, experiments that suggest the dissolution of the atom--that suggest, in short, that the Daltonian atom is misnamed, being a structure that may, under certain conditions, be broken asunder. But these experiments have, as yet, the warrant rather of philosophy than of pure science, and to-day we demand that the philosophy of science shall be the handmaid of experiment. When experiment shall have demonstrated that the Daltonian atom is a compound, and that in truth there is but a single true atom, which, combining with its fellows perhaps in varying numbers and in different special relations, produces the Daltonian atoms, then the philosophical theory of monism will have the experimental warrant which to-day it lacks; then we shall be a step nearer to the atom of Democritus in one direction, a step farther away in the other. We shall be nearer, in that the conception of Democritus was, in a sense, monistic; farther away, in that all the atoms of Democritus, large and small alike, were considered as permanently fixed in size. Democritus postulated all his atoms as of the same substance, differing not at all in quality; yet he was obliged to conceive that the varying size of the atoms gave to them varying functions which amounted to qualitative differences. He might claim for his largest atom the same quality of substance as for his smallest, but so long as he conceived that the large atoms, when adjusted together to form a tangible substance, formed a substance different in quality from the substance which the small atoms would make up when similarly grouped, this concession amounts to the predication of difference of quality between the atoms themselves. The entire question reduces itself virtually to a quibble over the word quality, So long as one atom conceived to be primordial and indivisible is conceded to be of such a nature as necessarily to produce a different impression on our senses, when grouped with its fellows, from the impression produced by other atoms when similarly grouped, such primordial atoms do differ among themselves in precisely the same way for all practical purposes as do the primordial elements of Anaxagoras.