LECTURE XV.] HISTORY OF CHEMISTRY. 31 1 



account the results of his researches on organic bases. The 

 same idea was more fully carried out by Weltzien, 70 H. Schiff, 71 

 Cleve, 72 and many others. 



Despite all this, it cannot be said that the theory of valency 

 has proved very productive in inorganic chemistry. In the 

 first place, the number of researches which it has occasioned 

 is by no means very considerable ; and, further, a systematic 

 treatment of the subject, based upon valency, is not capable 

 of being uniformly and logically carried out. Another hypo- 

 thesis has had a far more important and lasting effect, and is 

 now, some thirty years after its first promulgation, able to show 

 the most brilliant and undreamt-of results. I must now discuss 

 the relations which have been found to subsist between the 

 atomic weights and the properties of the elements. 



The more recent investigations on this subject are related 

 to Prout's hypothesis, which has already been considered. 73 It 

 is true that this hypothesis never became generally accepted, 

 but nevertheless it occasioned speculations from time to time 

 in the same direction. I only mention here Dobereiner, who, 

 iri 1829, first drew attention to what he called the triads 74 (that 

 is, to groups of three analogous elements possessing atomic 

 weights such that one of them might be regarded as the arith- 

 metical mean of the other two). Gmelin, 75 Dumas, 76 and 

 Lenssen 77 further elaborated these ideas, without arriving at 

 any results specially worth mentioning. 



A valuable result was, however, attained by the proof that 

 the properties of the elements are periodic functions of their 

 atomic weights. For this, we are indebted to the investigations 

 of Newlands, 78 Lothar Meyer, 79 and Mendelejeff. 80 The chief 

 merit unquestionably belongs to the latter, who first gave pro- 

 minence to the existing relations in a quite general form, and 



70 Ibid. 97, 19. 71 Ibid. 123, i. r2 Bull. Soc. Chim. [2] 7, 12 ; 15, 

 161 ; 16, 203 ; 17, 100, 294. 73 Compare p. 102. 74 Pogg. Ann. 15, 

 301. 75 Handbuch. Third Edition, I, 35. 76 See p. 102. 77 Annalen. 

 103, 121 ; 104, 177. 78 Chem. News. 10, 59, 94; 13, 113. 79 Moderne 

 Theorien. First Edition, 136 ; Annalen. Supplementband 7, 354. 80 Zeit- 

 schrift fiir Chemie. 12, 405 ; Annalen. Supplementband 8, 133. 



