270 HISTORY OF CHEMISTRY. [LECTURE XIII. 



with which the substances can combine with hydrogen, 

 chlorine, hypochlorous acid, etc. The capacity for entering 

 into direct additions thus became a characteristic of the group ; 

 but it cannot be said to be really distinctive, since some 

 substances which are classed as saturated also possess this 

 capacity. As examples of the latter substances the aldehydes 

 and ketones in particular may be instanced, and these are 

 substances which contain oxygen wholly united to carbon. In 

 explanation of the facts, the assumption is made regarding 

 these compounds that, by addition, the group (C = O)" passes 

 into (C O)"" ; that is to say, a diatomic radical becomes a 

 tetratomic one. Later experiments of a very detailed char- 

 acter on the unsaturated acids, by Fittig, have led to the 

 confirmation of the view mentioned above ; 83 that is, they have 

 shown that the facts are best accounted for when blanks, or 

 bivalent carbon atoms, are assumed in some compounds at 

 least. That it is not possible to avoid some assumption of this 

 kind is shown by carbonic oxide and by the group of isonitriles 

 or carbylamines, discovered almost simultaneously by Hofmann 84 

 and by Gamier. 85 The latter interesting substances are 

 obtained by treatment of the amines with chloroform and by 

 the action of the alkyl "iodides on silver cyanide. They are 

 isomeric with the nitriles, and their constitution cannot be 



f R 



represented otherwise than by the formula N p, first proposed 



by Gautier, 80 in which R stands for a monatomic alcohol 

 radical. If the nitrogen is assumed to be trivalent, 87 the 

 carbon then appears as bivalent or unsaturated. 



There is, besides, a class of unsaturated substances, in which, 

 following Kekule's lead, a more intimate union of the carbon 

 atoms is quite generally assumed. I refer to the aromatic com- 

 pounds. Under this heading a number of substances which 

 stand in a close chemical relationship to certain strongly smell- 

 ing oils were formerly grouped together. 



83 Annalen. 188, 95. 84 Ibid. 144, 114 ; 146, 107. 8r> Comptes 

 Rendus. 65. 468 ; Annalen. 146, 119. 86 Comptes Rendus. 65, 901. 



87 Quinquivalent nitrogen is referred to in Lecture 15. 



