246 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



obtained by heating mercuric cyanide (loc. cit. pp. 172-199). 

 As one of the first examples of a " compound radical," it 

 became of dominant importance in the development of 

 structural chemistry during the ensuing forty years. 



E. HYDRACIDS AND HALOGENS. 



Gay-Lussac (1814) abandons the oxygen-theory of 

 acids. The discovery of hydriodic acid, confirming Davy's 

 contention that muriatic acid was a simple hydride, con- 

 taining no oxygen, made it necessary to abandon the 

 limitations imposed by Lavoisier's oxygen-theory of acids. 

 Gay-Lussac was impressed by the fact that sulphur, chlorine, 

 and iodine could form acids by combining either with 

 oxygen or with hydrogen. He suggested that these com- 

 pounds should be distinguished as follows : 



Oxygen-Acids Hydrogen- Acids 



SULPHURIC ACID HYDROSULPHURIC ACID 1 



CHLORIC ACID HYDROCHLORIC ACID 



IODIC ACID HYDRIODIC ACID. 



" Like [sulphur and chlorine, iodine] forms two acids, the 

 one by combining with oxygen, and the other by combining 

 with hydrogen. . . . The compounds of chlorine, of 

 iodine and of sulphur with hydrogen, which show the 

 properties of the acids formed by oxygen, must be included 

 in the same class under the same name of acid ; but to 

 distinguish them, I propose to add to the specific name of 

 the acid, the generic word hydro \ so that the acid compounds 

 of hydrogen with chlorine, iodine and sulphur, would bear 

 the name of hydrochloric acid, hydriodic acid, and hydro- 

 sulphuric acid ; and the acid compounds of oxygen with the 

 same substances, would have, according to the recognised 

 principles of nomenclature, the names of chloric acid, iodic 

 acid, etc." (Ann. de Chimie, 1814, 91, 8-9). 



1 i.e. , sulphuretted hydrogen. 



