CLASSIFICATION OF THE ELEMENTS As early as 1815 Prout put forward the view that all the elements were compounded of hydrogen, and all the atomic weights were simple multiples of that unit. That so many of the atomic weights approached whole numbers very closely seemed to exclude that this was a mere chance coincidence ; but on the other hand there were well-established cases, such as chlorine, where the weight could not be reconciled with Prout's hypothesis. It was only after Cannizzaro's reform that a "natural" arrange ment of the elements became possible. A scheme due to de Chan courtois in 1862 was followed by the law of Octaves published by Newlands in 1864. The latter placed all the known elements in eight vertical columns of seven each in the order of their weights, the second column starting with fluorine, the third with chlorine, and so on. Neither of these arrangements was accepted by chemists, although they both embodied the idea that carried conviction when put forward, quite independently, by Mendeleyev in 1869.
In the Periodic Classification (q.v.) Mendeleyev showed how regularly the elements changed in character as they changed in weight until the end of each "period" was reached, when by a sudden leap the character of the first was reproduced. In some doubtful cases he assigned atomic weights so that the elements should fit their places. Moreover he left many blank places to be filled, and was able to predict in some cases the atomic weight and the chief properties of the undiscovered element. His pre dictions were most startlingly fulfilled in the discovery of gallium, scandium and germanium.
The discovery of the inert gas, argon, by Rayleigh and Ram say in 1894, and their proof that its molecule contained a single atom only, appeared at first at variance with the Periodic System.
But when Helium was discovered Ramsay was able to place these two gases as transitional elements and to predict another member of this family with an atomic weight between those of fluorine and sodium. By an exhaustive series of fractional distillations of the argon residues from air he and his colleagues separated first Neon and finally the two heavier gases Krypton and Xenon. These gases, like helium and argon, are chemically inert, and like them are monatomic. The chemical inertness of these elements has been explained, on the electron theory, by the symmetry of the orbits of their valency-electrons.
The proved success of the Periodic System both in co-ordination and prediction of chemical facts naturally led to the conviction that either its "weight" or its "place" held the secret of an ele ment's properties. Many attempts were made to show "multiple" relations between the atomic weights, but no real advance was made until new discoveries in physical chemistry gave fresh views as to the ionization of salts in aolution, the nature of electric dis charges in gases, and the mechanism of radioactivity (q.v.).