248 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



" What we call a salt must be defined according to a 

 certain electro-chemical relation and without any regard to 

 the number of the constituent elements. Thus we say that 

 the compound of chlorine with sodium is a salt, because 

 these two substances completely neutralise the electro- 

 chemical properties of one another. . . . 



u According to this view, electro-negative substances may 

 be divided into three classes. First : those which form 

 salts by neutralising the electro-positive metals : I call these 

 halogens (salt-producers) ; they are chlorine, iodine, and 

 fluorine. . . ." 



" Salts may be divided into two classes : those which 

 result from the combination of a halogen with an electro- 

 positive metal, and those which are formed by the union 

 of an acid with a base : I will call the first haloid-salts, 

 and the second amphi-salts" ("Memoir on Sulpho-salts," 

 Ann. Chim. Phys., 1826, 32, 62 63 and 65). 



In the second class of salts Berzelius included not only oxy- 

 salts formed from two oxides, \mi"sulpho-salts formed from 

 two sulphides, etc. The discovery of bromine in the 

 following year completed the series of four halogen-elements 

 (fluorine, chlorine, bromine and iodine), and added a new 

 group of haloid-salts, but did not lead to any further 

 theoretical developments. 



The hydrogen-theory of acids. The uniformity of com- 

 position, which was implied in Lavoisier's oxygen-theory of 

 acids, was destroyed by the discovery of the hydracids. It 

 was restored later by a theory which regarded all acids as 

 hydrogen compounds. 



The " hydrogen theory " of acids was foreshadowed, in 

 1816, by Davy (Works, V. 514 515), who regarded it as 

 "a simple statement of facts to say that liquid nitric acid is 

 a compound of ... hydrogen . . . azote and . . . oxygen," 

 and suggested that " hydrogen in [an] acid, may be con- 

 sidered as acting the part " played by a metal in the salts 

 derived from it. 



