162 ESSAYS BIOGRAPHICAL AND CHEMICAL 



After the convenience of Dalton's hypothesis had been 

 acknowledged, the labour of chemists was for many years 

 devoted to the determination of the relative values of the 

 ' atomic weights ' of the elements ; or, expressed in a 

 manner independent of hypothesis, of their combining 

 proportions. The name of the Swedish chemist, Berzelius, 

 is prominent in this connection. By the analysis of an 

 almost incredibly large number of compounds, he estab- 

 lished on a firm basis the constancy of composition of 

 compounds, and the law of multiple proportions. Towards 

 the 'forties, therefore, a set of numbers had been col- 

 lected, which invited an attempt to place them in order, 

 with the view of seeing whether some still more profound 

 law could not be discovered connecting the combining 

 numbers attached to them. Dobereiner, as early as 1817, 

 and again in 1829, pointed out that certain elements had 

 atomic weights which were nearly the mean of those of 

 others which were closely related to them ; thus, the mean 

 of the atomic weights of calcium and barium gives a close 

 approximation to the atomic weight of strontium ; that of 

 sodium lies near the mean of those of lithium and potassium; 

 and sulphur and tellurium similarly indicate selenium 

 as a middle element. In 1843 Gmelin, who published 

 a Handbook of Chemistry, which is still a classic, attempted 

 a classification based, not upon numerical relations, but 

 on similarity of properties. For instance, we find the 

 groups F, Cl, Br, I; S, Se, Te; P, As, Sb; C, B, Si; Li, 

 Na, K ; Mg, Ca, Sr, Ba ; and so on. In 1851 Dumas gave 

 a lecture before the British Association, in which he 

 showed that not merely is the atomic weight of bromine 

 the mean of those of chlorine and iodine, but that its 

 physical properties, such as its colour, its density in the 

 gaseous and in the liquid state, etc., are also half-way 

 between those of the allied elements. In 1852 Faraday 

 criticised Dumas' attempts as 'speculations which have 



