LECTURE XII.] HISTORY OF CHEMISTRY. 241 



power of holding together several atomic groups. There were 

 but few who then understood the meaning which lay in 

 Williamson's words, and likewise few who recognised the 

 extension that might be given to them. Amongst these, it 

 was Kekule in particular, whose sagacity led him to perceive 

 at once the bearing of Williamson's ideas, and who made use 

 of these ideas to elucidate the relations of thioacetic acid 

 which he had discovered in i853. 58 - 



Kekule compares the reactions of phosphorus pentachloride 

 and of phosphorus pentasulphide upon acetic acid, writing : 



[C=i2,0=i6, 6 = 32, etc.] 



Referring to this he remarks: "The above diagram . . . 

 exhibits ... the relations between the reactions obtained with 

 the chlorine and with the sulphur cojnpounds of phosphorus. 

 In fact, it is perceived that the decomposition is, in its essential 

 features, the same; only, when the chlorides of phosphorus are 

 employed, the product breaks up into chlorothyl [C 2 H 3 OC1] 

 and hydrochloric acid, . . . whereas when the sulphur com- 

 pounds of phosphorus are employed, both groups remain 

 united, because the quantity of sulphur which is equivalent to 

 the two atoms of chlorine is not divisible." 



These points of evidence lead Kekule to declare in favour 

 of the accuracy of the "new atomic weights " (Gerhardt's equi- 

 valents). These, according to Kekule, are a better expression 

 of the facts than the mode of representing them previously 

 in use. Even if the new formulae are adopted, and the old 

 equivalents retained, it cannot be perceived why phosphorus 

 sulphide produces mercaptan from alcohol whilst phosphorus 

 chloride forms ethyl chloride and hydrochloric acid (CfL^C 1 

 and HC1); why the latter do not remain combined just as 

 C 4 H 5 S and HS do, etc. It is not merely a difference in the 



58 Annalen. 90, 309. 

 Q 



