144 MORRIS LOEB 



of exact coincidences with Prout's law, as compared with 

 that of deviations from it, is not much greater than what 

 would be expected 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 approximately 

 in arithmetical progression, as lithium (7), sodium (23), and 

 potassium (39); calcium (40), strontium (88), and barium 

 (136) ; sulphur (32), selenium (79), and tellurium (127) ; chlo- 

 rine (35.5), bromine (80), 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 con- 

 stant differences were in many cases multiples of 16, the atomic 

 weight of oxygen, whence it was assumed that the heav- 

 ier elements of a group might be oxides of the lightest, thus 

 reducing the number of primordial elements considerably. 

 The idea of connecting all the atomic weights in a single 

 progression wherein similar elements recurred at regular 

 intervals seems to have first struck de Chaucourtois, and 

 shortly afterwards Newlands; but the law in its complete 

 form is due to Mendeleeff and Lothar Meyer, who reached 

 the same conclusion independently in 1869. As Mendeleeff's 

 exposition was by far the more convincing, he has been given 

 the greatest share of the credit. 



A good idea of the fundamental principle can be obtained 

 from the accompanying figure, in which the maximum va- 

 lencies in the elements and their melting-points are shown 

 to be periodically related 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 be seen 

 that the two curves connecting the respective points are un- 



