A HUNDRED YEARS OF CHEMISTRY. 683 



siiniiltaneoiislv Lothar Meyer annoimced similar views, but inde- 

 pendently, and controversy soon arose as to the relative merits of 

 tlie two philosopliers. With, that controversy we have nothing to 

 do, but the law itself deserves our fuller attention. 



According to the periodic law, all the properties of the elements 

 are periodic functions of their atomic weights, varying from sub- 

 stance to substance in a perfectly regular manner. The elements 

 thus fall into periods, or octaves, as i^ewlands called them, of a 

 very striking character. If, for example, we start with univalent 

 lithium, the next higher element has a valence of two, the next of 

 three, and then comes carbon, whose atom is quadrivalent. Fol- 

 lowing carbon, the combining power of successive elements decreases 

 until we reach sodium, in which something like the properties of 

 lithium recur. Above sodium the same rise goes on to the fourth 

 element higher, silicon, which resembles carbon, and then follows 

 the regular step-by-step falling away, to end with chlorine, the last 

 member of the second period. This periodic rising and falling is 

 characteristic of all the elements, and they were so tabulated by 

 Mendelejeff as to be perfectly clear, with a clearness which is not to 

 be given by words. In Mendelejeff's table certain gaps appeared, 

 which he ascribed to the existence of undiscovered metals. Tor 

 three of these he predicted the properties, starting out from the 

 properties of their neighbors. This was a rash thing to do, but 

 the venture has been fully vindicated. In 1875 Lecoq de Bois- 

 bandram discovered gallium, which filled one of the gaps; scandium 

 and germanium filled the other two later. The predictions of Men- 

 delejeff were fulfilled; atomic weight, specific gravity, fusibility, 

 the character of the compounds to be formed, were all foreseen 

 for each of the three new elements; and, so far as experiment has 

 yet gone, his anticipations have been perfectly realized. Every 

 good theory is prophetic; but few generalizations have been so 

 strikingly verified in this respect as has the periodic law. In spite 

 of some outstanding difficulties, yet to be explained, the law has 

 served to great advantage in the classification of the elements, and 

 it has had much to do with the late revival of inorganic chemistry. 

 The latter branch of science, long comparatively neglected, has 

 now gained new interest, and for it, in the near future, a great 

 growth can be prophesied. 



The immediate effect of the periodic law was to prove that the 

 elements are connected with one another by general relations, and 

 so to stimulate the belief in their possibly common origin. This 

 view has many upholders, although it is also strongly opposed, but 

 the weight of argument seems to b^ in its favor. On philosophic 

 grounds it is at least more' probable than the opposite opinion, 



