LECTURE XV.] HISTORY OF CHEMISTRY. 309 



quinquivalent or septivalent it is possible for them to include 

 in the system a large number of molecular compounds. But it 

 also becomes necessary for the adherents of this view to explain 

 the change in the saturating capacity, or at least to establish 

 the conditions which bring about this change in the properties 

 of the elements, if their hypotheses are to deserve the name of 

 a theory. Very little has yet been done in this direction, how- 

 ever, and the little that has been done is scarcely capable of 

 any general formulation. 01 On the other hand, a number of 

 facts have become known which can only be explained with 

 difficulty on the assumption of constant valency ; such, for 

 example, as the identity of the naphthyl-phenyl sulphones and 

 of the tolyl-phenyl sulphones which can be prepared in dif- 

 ferent ways ; 62 or such as the isomerism of the two triphenyl- 

 phosphine oxides, one of which, P(C (5 H 5 ) 3 O, is supposed to 

 correspond to phosphorus pentachloride ; and the other, 

 P(C 6 H 5 ) 2 OC 6 H 5 , to phosphorus oxychloride. 63 



It is clear from these few observations that the subject of 

 valency, quite apart from any mathematical basis (which is at 

 present altogether wanting 64 ), must still be called a very anoma- 

 lous and uncertain one, and that there is no existing conception 

 of it which is capable of dealing in a logical manner with the 

 whole domain of chemistry. 



That the idea is still retained, in spite of this, and that it is 

 even yet regarded as one of the most important principles, is 

 explicable, in organic chemistry at least, on account of the 

 almost marvellous consequences which the latter branch of the 

 subject is able to show as the result of its assistance during the 

 last forty years. In inorganic chemistry, however, the state of 

 matters is very different. 



No doubt a favourable and helpful influence may be ob- 

 served in inorganic chemistry also ; and, in particular, classifi- 



61 Compare, however, Horstmann, Theoretische Chemie. 327 et seq. ; 

 Van 't Hoff, Ansichten liber die Organische Chemie. I, 3. 62 Michael 

 and Adair, Berichte. 10, 583; II, 116. 63 Michaelis and Lacoste, 



ibid. 18, 2118. 64 Compare, however, Kekule, Annalen. 162, 86; 



Baeyer, Berichte. 18, 2277. 



