they failed to recognize the atom through all its migrations. Thus a meal of bread and water contains exactly the same kind of atoms as a draught of a solution of prussic acid; by merely mixing two colourless liquids we can get another showing the most vivid colour; iron is intensely magnetic, so are many of its salts; there are others however which, as Professor Townsend has shown, are non-magnetic, while some of those interesting compounds of iron and carbon monoxide are actually diamagnetic. Does the atom then preserve nothing intact as it goes from one compound to another except its weight? We now know that it does, and we can now give convincing proof of the individuality of the atom throughout migration. The visible light which the atom emits changes with the compound, yet, as Professor Barkla has shown, an atom besides this visible light can also emit that peculiar kind of invisible light called Röntgen rays, which only differs from ordinary light in the kind of way that blue light differs from red. Barkla has shown that each kind of atom emits a peculiar type of Röntgen ray, which remains unaltered, whatever kind of partner the atom may have. Thus we can detect the presence of iron, say, in any compound, by studying the Röntgen rays emitted by that compound; if it contains iron we shall find the characteristic Röntgen radiation of iron present, however complex the compound may be. With such penetrating agents as Röntgen and cathode rays at our disposal, other properties which the atom retains unaltered have been brought to light, such, for example, as the absorption of these rays when they pass through atoms; the absorption by a given atom is quite independent of any other atoms with which it may happen to be associated, and depends only on the quality of the atom itself.