APPENDIX II. 47$ 



ench elements as Ba and V, Co and N.i, or Rh and Ru, occupied one place in 

 the octave, 3 Nevertheless, the fruit was ripening, and I now see clearly thafr 

 Strecker, De Chancourtois, and Newlands stood foremosf in the way towards 

 the discovery of the periodic law, and that they merely wanted the boldness 

 necessary to place the whole question at such a height that its reflection on 

 the facts could be clearly seen. 



A third circumstance which revealed the periodicity of chemical elements 



, ivas the accumulation, by the end of the sixties, of new information respecting 



' the rare elements, disclosing their many-sided relations to the other elements 

 and to each other. The researches of Marignac on niobium, and those of 



, Roscoe on vanadium, were of special moment. The striding analogies between 

 vanadium and phosphorus on the one hand, and between vanadium and 

 chromium on the other, which became so apparent in the investigations con- 

 nected with that element, naturally induced the comparison of V = 51 with 

 Cr = 62, Kb = 94 with Mo = 96, and Ta = 192 with W = 194; while, on the 

 other hand, P = 31 could be compared with S 32, As 75 with Se - 79, and 

 Sb 120 with Te 125. From such approximations there remained but one 

 Step to the discovery of the law of periodicity. 



The law of periodicity was thus a direct outcome of the stock of generali- 

 sations and established facts which had accumulated by the end of the decade 

 1860-1870 : it is an embodiment of those data in a more or less systematio 

 expression. \Vhere, then, lies the secret of the special importance which has 

 since been attached to the periodic law, and has raised it to the position of a 

 generalisation which has already given to chemistry unexpected aid, and 

 which promises to be far more fruitful in the future and to impress upon 

 several branches of chemical research a peculiar and original stamp? The 

 remaining part of my communication will be an attempt to answer this 

 Question. 



In the first place we have the circumstance that, as soon as the law made 

 its appearance, it demanded a revision of many facts which were considered 

 by chemists as fully established by existing experience. I shall return, later 

 on, briefly to this subject, but I wish now to remind you that the periodic 

 law, by insisting on the necessity for a revision of supposed facts, exposed 

 itself at once to destruction in its very origin. Its first requirements, how- 

 ever, have been almost entirely satisfied during the last 20 years ; the sap- 

 posed facts have yielded to the law, thus proving that the law itself was a 

 legitimate induction from the verified facts. But our inductions from data 



'bave often to do with such- details of a s6ience so rich in facts, that only 

 generalisations which cover a wide range of important phenomena can attract 

 general attention. What were the regions touched on by the periodic Jw ? 

 This is what we shall now consider. 



The most important point to notice is, that periodic functions, used for 

 the purpose of expressing changes which are dependent on variations of time 

 find space, have been long known. Tbey are familiar to the mind when we 

 have to deal with motion in closed cycles, or with any kind of deviation from 



* To judge from J. A. R. Newlands's work, OH the Discovery of the Periodic Law t 

 London, 3884, p. 149; 'Oo the Law of Octaves' (from the Chemical News, 12, 83, 

 . 1S65J. 



