124 HISTORY OF CHEMISTRY. [LECTURE VII. 



symbolic mode of expression, which cannot be regarded as 

 representing the actual composition of the substances. Only 

 some years afterwards does he revert, for a short time, to 

 Dumas' ideas, and he then calls the radical C 4 H 8 Etherin. 47 



This appears to me to be the place to state the results of 

 an investigation of Gay-Lussac's into the cyanogen compounds, 

 which had been carried out as early as 1815, and contributed 

 materially to giving a more definite meaning to the conception 

 of a radical. 48 Gay-Lussac repeated Berthollet's experiments 

 on the composition of hydrocyanic acid and confirmed them, 

 inasmuch as he established beyond all doubt that the acid is 

 free from oxygen, and contains carbon, nitrogen, and hydrogen 

 only. The examination of the salts led him to study the 

 behaviour of mercuric cyanide at high temperatures, and thus 

 to discover cyanogen. What is of importance for us in Gay- 

 Lussac's work, is the way in which he regards the substances 

 he describes. These are, in his view, compounds of a radical 

 containing carbon and nitrogen (cyanogen) and identical with 

 the gas obtained from mercuric cyanide. The possibility of 

 preparing radicals was in this way demonstrated, and, in con- 

 sequence, the conception attained a more real significance. 

 It is further to be remarked that Gay-Lussac, in calling the 

 radical of hydrocyanic acid, cyanogen, permitted himself a 

 certain freedom, since it was not actually " the residue of an 

 acid which has been deprived of its oxygen." Obviously the 

 great French scientist compares hydrocyanic acid with hydro- 

 chloric acid, and with hydriodic acid which he had himself 

 discovered a short time previously. They are hydrogen com- 

 pounds of elements or radicals, exactly as the ordinary acids 

 are oxygen compounds. If it was now desired to define a 

 radical, and to include cyanogen in the definition, it was no 

 longer possible to say, with Lavoisier, that " it is the residue 

 of a substance which has been deprived of its oxygen " ; but 

 it was the other half of the definition that was to be accen- 

 tuated; "a radical is a composite group which behaves like an 



47 Annalen. 3, 282. 48 Ann. Chim. 95, 136. 



