sodium, potassium, rubidium and cæsium, all white, soft metals, all easily oxidizable, all at once violently attacked by water, and generally with such energy as to be inflamed at the contact. It required no great penetration to class such elements as these into classes.
The revival of the hypothesis of the atomic constitution of matter by Dalton and of his attempt to determine the atomic weights of the elements was not long in provoking the guess that perhaps there could be found some connection between the numbers representing the relative atomic weights of kindred elements. But, as is well known, the state of knowledge in Dalton's day was not sufficiently advanced to enable him to attribute to elements their correct relative atomic weights; and it was not until the eminent professor of chemistry in Rome, Cannizzaro, whose jubilee has recently been celebrated, pointed out the bearing on Dalton's numbers of all the facts accumulated up to the year 1856 that the close relationship between the atomic weights and the properties of the elements was suggested by John Newlands. Some years later, Lothar Meyer and Dmitri Mendeléef amplified and elaborated the ideas which had first been propounded by Newlands; and the periodicity of the atomic weights and the gradual variation of the properties of the elements and their compounds were established on a firm basis.
Various plans have been adopted to render this arrangement pictorially visible; each method has perhaps its own conveniences, but none can be regarded as the method par excellence. Lothar Meyer's original table is constructed on the hypothesis that a cylinder, on which the numbers have been distributed in their order on a descending spiral in eight main columns, has been unrolled.
Another method of representation is due to Dr. Johnstone Stoney. The atomic weights are represented on a spiral curve, closely approximating in form to a logarithmic spiral, and the magnitudes of the atomic weights are represented by the volumes of concentric spheres. Thus, the sphere in the middle stands for unity, the atomic weight of hydrogen. The elements follow each other according to the numerical order of their atomic weights; and by joining the points thus obtained, a nearly regular spiral curve is produced, resembling one derived by aid of a logarithmic or elliptical formula. The deviations from regularity appear also to follow a law, and if accurately mapped the spiral is a sinuous one. But the determination of individual atomic weights is as yet not sufficiently accurate to make it possible to calculate the course of the wavy line.
A third diagram, modified from that of Meyer, has been constructed by Professor Orme Masson, of Melbourne. The chief difference is that instead of grouping elements of the iron, palladium and platinum groups, they are distributed; hydrogen forms the first element of the
