xii THE HALOGENS 247 



Gay-Lussac on Hydracids. In a section on Acidity and 

 Alkalinity, appended to his " Researches on Iodine," Gay- 

 Lussac urged, as a general proposition, that the name " acid " 

 must be extended to include a series of acidic hydrides, 

 the HYDRACIDS, in addition to the well-known OXYACIDS. 



" For a long time I have regarded an acid, in the most 

 general sense of the word, as merely a substance, containing 

 oxygen or not, which neutralises alkalinity, and an alkali 

 likewise is only a body which neutralises acidity. 



" Whatever may be the definition of acids at which one 

 arrives, they must be formed into several groups, because 

 they do not derive their acid character from the same- 

 substance. We have : 



" i. The acids properly so-called, in which oxygen may be 

 considered as the acidifying principle, and which contain 

 only two elements ; such as ... sulphuric, sulphurous, 

 . . . carbonic acids. 1 



" 2. Acids formed by hydrogen and another substance : 

 this group includes the hydrochloric, hydriodic, and hydro- 

 sulphuric 2 acids. It is probable that in these acids, chlorine 

 iodine and sulphur are the acidifying principles ; but as 

 hydrogen is present in all of them, I thought it was more 

 convenient to derive their generic name from it. These 

 different acids can be designated by the name of 

 hydracids" (Ann. de Chimie^ 1814, 91, 145, 147, 148). 



Berzelius (1825) on halogens and haloid-salts. Gay- 

 Lussac' s dual classification of the acids was accepted by Ber- 

 zelius. It carried with it the recognition of two types of salts, 

 since the union of two oxides produced a ternary salt, whilst 

 the interaction of an acidic hydride with a basic oxide gave rise 

 to a binary salt and water. He therefore proposed in 1825 3 

 to describe the binary compounds as HALOID SALTS, and 

 their non-metallic components as HALOGENS. 



1 i.e., sulphuric, sulphurous, and carbonic anhydrides. 



2 i.e., sulphuretted hydrogen. 



3 The Swedish paper was translated into French in 1826. 



