256 HISTORICAL INTRODUCTION TO CHEMISTRY CH. xn 



the symbol Cy being used as an abbreviation for the cyan- 

 ogen radical CN. Gay-Lussac prepared cyanogen gas by 

 heating mercuric cyanide : 



E. HYDRACIDS AND HALOGENS 



Iodine was recognised as an element both by Davy and 

 by Gay-Lussac ; its acid hydride was therefore a binary 

 compound, containing no oxygen. Gay-Lussac (1814) pro- 

 posed to divide the acids into two groups, namely : 



(1) the oxy-acids (SO 2 , SO 3 , CO 2 , etc.), and 



(2) the hydro-acids, including in the latter group hydrochloric 

 acid (HC1), hydriodic acid (HI), and " hydro sulphuric acid" 

 (sulphuretted hydrogen, H 2 S) ; hydrofluoric acid (HF) and 

 hydrocyanic acid (prussic acid, HCN) were soon added to the 

 second group of acids. 



Berzelius (1825) recognised the existence of two correspond- 

 ing series of salts, namely : 



(1) amphi- salts, which were ternary compounds, formed by 

 the union of an acidic and a basic oxide, sulphide or selenide, and 



(2) haloid-salts (e.g. common salt, NaCt), which were 

 binary compounds of a metal with a halogen, fluorine, chlorine, 1 

 iodine, etc. 



Laurent (1854), following Davy (1816), regarded all acids 

 as salts of hydrogen ; sulphuric acid was then hydrogen 

 sulphate, H 2 SO 4 , whilst the oxide (SO 3 ) which had been 

 regarded previously as sulphuric acid was now called sulphuric 

 anhydride ; so, too, carbonic acid was H 2 CO 3 (compare Na 2 CO 3 ) 

 whilst the oxide (CO 2 ) was called carbonic anhydride. 



1 Bromine was not discovered until the following year. 



