270 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Daniell (1840), regards salts as binary compounds of two 

 radicals. Gay-Lussac, in 1815, had recognised in 

 cyanogen a compound-radical which could act the part of a 

 non-metallic element, such as chlorine, iodine or sulphur. 

 Ampere, in 1816, recognised in ammonium a compound 

 radical which could act the part of a metal. The way 

 was thus opened up for a final unification of all the different 

 types of salts. The binary salts formed by the union of 

 metal and halogen now became the type instead of the 

 exception. In the ammonium salts, the metal was replaced 

 by a compound-radical : in the cyanides, the halogen was 

 replaced by a compound-radical. It only remained to include 

 the oxygen salts in the same category by regarding the sul- 

 phates as compounds of a metal, with a compound 

 SULPHATE-RADICAL. This radical was composed of sulphuric 

 anhydride united with the oxygen of the base, thus : 



LEAD OXIDE = { LEAD = LEAD ' 



LEAD SULPHATE = SULpHURIC J* 1 SULPHATE . 



ANHYDRIDE ^SULPHUR J 



just as the ammonium of sal-ammoniac (p. 268) was com- 

 posed of ammonia united with the hydrogen of the acid. 



This view of the nature of salts was generally adopted 

 when Daniell, in 1840, showed that the first action of an 

 electric current on salt solutions was to decompose the salt 

 into metal and non-metal, if the radicals were simple, or if 

 the radicals were compound, into "metallic" and "non- 

 metallic "compound-radicals. 



B. DECOMPOSITION OF THE ALKALIES AND EARTHS. 



Volta (1790) produces an electric current by means of 

 the voltaic pile. The Italian physicist Volta discovered in 

 1790 that insulated discs of zinc and copper, when brought 

 into contact for a moment and then separated, acquired 



