PERIODIC LAW. The generally accepted embodiment of the relations existing between the various properties of the chemical elements. so far as they eau lie compered with one another. It may be stated as follows: If the elements are arrangid in the order of their atomic weights, each of their properties rarics as a periodic func tion of the atomic weight.
Ever since the work of Richter, Proust, and Dalton hail established the idea of fixed numerical values attaching to the ingredients of compounds. an idea which was deduced by Dalton from the hypothetical existence of individual atoms, iden tical in size, mass and other properties for any one element, chemists sought to deduce a closer relationship between the various elements from a comparison of the masses of their respective a toms. CHEMISTRY and ATOMIC WEIGHTS. ) The first attempt was that made by Dr. Prout in 1815 to prove that all the atomic weights were even multiples of the atomic weight of hydrogen and that the latter was the only primitive ele ment, from which the others were derived by processes of condensation. It was soon found that very few elements possessed atomic weights that could be expressed by integers, when the atomic weight of hydrogen was set at unity, and Prom's law was gradnally modified to state that one-hal f the atomic weight of hydrogen. then that one-quarter, should be taken as the real standard. Refinements of investigation have since estab lished the relative atomic weights to the second place of decimals, and it can now be asserted that the number of exact coincidences with Prout's law, as compared with that of deviations from it, is not much greater than what would be ex pected by the theory of chances. Prout's law has, therefore, been practically abandoned. On the other hand. interesting relations were found to exist between the atomic weights of similar elements. Thus Doebereiner established, in 1829, his so-called triads, sets of three. closely related elements whose atomic weights were approxi mately in arithmetical progression, as lithium (7), sodium (23). and potassium (39) ; calcium ' (40). strontium (88). and barium (136) ; sul
phur (32), selenium (79). and tellurium (127) : chlorine (35.5), bromine (SO), and iodine (127) : iron (56), nickel (57), and cobalt (58). These triads were later extended to include longer sets, and it was also pointed out that the constant differences were in many eases multiples of 16, the atomic weight of oxygen. whence it was as sumed that the heavier elements of a group might be oxides of the lightest, thus reducing the num ber of primordial elienients considerably. The idea, of connecting all the atomic weights in a single progression wherein similar elements re curred at, regular intervals seems to have first struck de Chaucourtois, and shortly afterwards Newlands; but the law in its complete torm is clue to and Lothar .Never, who reached the same conclusion independently in ISO. As exposition was by far the more con vincing. lie has been given the greater share of the credit.
A idea of the fundamental principle can be obtained from the accompanying figure, in which the maximum valeneies of the elements and their melting-points are shown to be periodically 'elated to the atomic weights. The latter are laid off as abscissas, and the valencies and melting points as ordinates, on perfectly arbitrary scales. It will he seen that the two curves connecting other. In the eighth group occur triplets of closely analogous elements to be di-eus.ed below. Arrangements into fifteen or more individual groups, in place of the twin and triple groups here shown., have been suggested, but not generally adopted. .Mention should also he made of the fact that this table can be constructed by writing the elements in the order of their atomic• weights along a screw-line of slight pitch upon the surface of a cylinder, and then, as it were, unrolling the cylinder. Various efforts have been made to con nect all the atomic by a graphic equation, which would provide for an arrangement on some other kind of a spiral curve, either on a plane or in space. but they have been only moderately successful.