APPENDIX II. 473 



1 The aim of this communication will be fuHy attained if I succeed in. 

 drawing the attention of investigators to those relations which exist between! 

 the atomic weights of dissimilar elements, which, So far as I know, have; 

 hitherto been almost completely neglected. I believe that the solution of 

 eome of the most important problems of our science lies in researches of this] 

 kind.' 



To-day, twenty years after the above conclusions were formulated, they 

 may still be considered as expressing the essence of the now well-known 

 periodic law. 



Reverting to the epoch terminating with the sixties, it is proper to indi- 

 cate three series of data without the knowledge of which the periodic law 

 could not have been discovered, and which rendered its appearance natural 

 and intelligible. 



In the first place, it was at that time that the numerical value of atomic 

 weights became definitely known. Ten years earlier such knowledge did not! 

 exist, as may be gathered from the fact that in 1860 chemists from all parts' 

 of the world met at Karlsruhe in order to come to some agreement, if not 

 with respect to views relating to atoms, at any rate as regards their definite- 

 representation. Many of those present probably remember how vain were! 

 the hopes of coming to an understanding, and how much ground was gained 1 

 at that Congress by the followers of the unitary theory so brilliantly repre- 

 sented by Cannizzaro. I vividly remember the impression produced by his 

 speeches, which admitted of no compromise, and seemed to advocate truth 

 itself, based on the conceptions of Avogadro, Gerhardt, and Regnault, which 

 >Bt that time were far from being generally recognised. And though no 

 understanding could be arrived at, yet the objects of the meeting were attained,, 

 for the ideas of Cannizzaro proved, after a few years, to be the only ones 

 which could stand criticism,- and which represented an atom as 'the 

 smallest portion of an element which enters into a molecule of its compound.* 

 Only such real atomic weights not conventional ones could afford a basis 

 for generalisation. It is sufficient, by way of example, to indicate the 

 following cases in which the relation is seen at once and is perfectly clear : 



K = 39 Rb = 85 Cs=rl33 



Ca = 40 Sr=87 Ba = 137 



whereas with the equivalents then in use 



K =39 Rb = 85 Cs =133 



Ca = 20 Sr =43'5 Ba = 68'5 



the consecutiveness of change in atomic weight, which with the true values 

 is so evident, completely disappears. 



Secondly, it had become evident during the period 1860-70, and even 

 during the preceding decade, that the relations between the atomic, weights 

 of analogous elements were governed by some general and simple laws. 

 Cooke, Cremers, Gladstone, Gmelin, Lenssen, Petfcenkofer, and especially 

 Dumas, had already established many facts bearing on that view. Thus 

 Dumas compared the following groups of analogous elements with organic 

 radicles : 



