Page:Popular Science Monthly Volume 59.djvu/593

Rh fluorine column; and the long and short periods are kept separate by a fold in the diagram. A diagonal line, too, divides the 'metallic' from the 'non-metallic' elements.

Other devices have been suggested in order to represent diagrammatically the relations between the atomic weights; but it must be borne in mind that whatever system is employed, such plans are merely aids to thought, and have no real significance. They are on a par with the representation of numerical relations as curves, and can convey nothing which is not already contained in the actual numbers.

It was Mendeléef who first drew attention to the progressive alteration of the valency of the elements in passing from left to right along the table. While the metals of the alkalies, lithium, sodium, potassium, rubidium and cæsium, are all monads, in as much as one atom of any one of these elements is able to replace one atom of hydrogen, the typical monad, elements of the beryllium group are dyads; hence while the formula of sodium chloride is NaCl, that of calcium chloride is CaCl2, that of boron chloride, BCl3, for boron is a triad; the chloride of the tetrad, carbon, CCl4, and so on. And considering the compounds with hydrogen, where these exist, we have BH3, corresponding to the chloride; CH4, NH3, OH2, and finally ClH and FH. As the valency alters by unity in each ease, it appeared reasonable to place the elements on the table in equidistant columns; or, as in Dr. Stone/s diagram, on equidistant lines, dividing the spiral curve into eight equal segments.

Meyer, however, showed that if the elements be mapped on square

paper, so that the vertical divisions correspond to the volumes occupied by unit weight of the solid or liquid elements, while the horizontal divisions correspond with the atomic weights, a certain amount of