582 POPULAR SCIENCE MONTHLY. 



sodium, potassium, rubidium and caesium, all white, soft metals, all 

 easily oxidizable, all at once violently attacked by water, and gen- 

 erally 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 rela- 

 tive atomic weights of kindred elements. But, as is well known, the 

 state of knowledge in Dalton's day was not sufficiently advanced to en- 

 able him to attribute to elements their correct relative atomic weights; 

 and it was not until the eminent professor of chemistry in Eome, 

 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 Mendeleef amplified and elabo- 

 rated 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 pic- 

 torially 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 approxi- 

 mating in form to a logaritlunic 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 calcu- 

 late 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 



