316 HISTORY OF CHEMISTRY. [LECTURE XV. 



cation has become essentially clearer, as I wish to illustrate in 

 individual cases. The possibility of classifying the elements 

 themselves according to their valencies indicates a step in 

 advance, since analogies were thereby brought out which had 

 only been partially recognised previously. The analogy of 

 carbon with silicon had already been pointed out, but boron had 

 also been placed along with these two. The analogy of the two 

 former was now established much more clearly, whilst boron 

 was recognised as belonging to another series altogether. On 

 the other hand, titanium, zirconium, and tin were classed along 

 with carbon and silicon. Similarly arsenic, antimony, and bis- 

 muth took their places beside nitrogen and phosphorus, then 

 vanadium also, as a consequence of Roscoe's careful investiga- 

 tion, 65 and, finally, niobium and tantalum when the results of 

 Marignac's researches were published. 66 A similar thing took 

 place with the metals, which had hitherto been arranged either 

 according to their relative densities or to their analytical be- 

 haviour. The theory of valency exercised, further, a decided 

 influence upon the views respecting many classes of compounds. 

 This was the case with the silicates in particular. Wurtz showed 

 how the facts ascertained by him with respect to the condensa- 

 tions of glycol might be extended to the derivatives of silicic 

 acid, 67 and, by so doing, he brought sudden light into a hitherto 

 obscure region. Soon afterwards this region was further illumi- 

 nated by Tschermak's 68 important research on the felspars, in 

 accordance with which these substances must be regarded as 

 isomorphous mixtures of orthoclase, albite, and anorthite. The 

 numerous metal-ammonia and metal-ammonium compounds 

 now found a place in the system also, being looked upon as 

 ammonia or as ammonium chloride with hydrogen atoms re- 

 placed by metal or metallic oxide. Hofmann was the first to 

 attempt this classification, 69 and in doing so, he turned to 



65 Annalen. Supplementband 6, 77. ^ Ann. Chim. [4] 8, 5 and 49 

 (abstract); Annalen. 135, 49; Ann. Chim. [4] 9, 249; Annalen. Supple- 

 mentband 4, 350. 67 Repert. de Chimie pure. 2, 449 ; Lemons de 

 philosophic chimique. 181. m Pogg. Ann. 125, 139. 69 Annalen. 

 78, 253; 79, ii. 



