ii THE ACIDS 21 



Black in 1755 prepared the ACETATE OF MAGNESIA, in 

 addition to the muriate, nitrate, and sulphate, for comparison 

 with the corresponding salts of lime. 



Classification of salts. The salts described above were 

 prepared by the action of acids (i) on metals such as iron 

 and copper ; (2) on the alkalis, soda and potash ; (3) on 

 earths such as lime and magnesia. The distinction, which 

 was at first made between these three classes of salts, was 

 rendered of little value by the observation that the salts of 

 metals could be prepared much more easily from the earthy 

 calces which are formed when the metals are burnt : Glauber, 

 for instance, showed that the muriate of copper was prepared 

 easily by the action of muriatic acid on the calcined metal, 

 although the metal itself was attacked but slowly. The 

 distinction was broken down finally at the commencement 

 of the nineteenth century by the discovery that the alkalis 

 and earths were themselves calces of easily-burnt metals. 

 When this had been proved the custom arose of describing 

 all salts as derivatives of metals. Thus gypsum, which was 

 called by Lavoisier " sulphate of lime," is now described as 

 CALCIUM SULPHATE, and Glauber's salt is called SODIUM 

 SULPHATE instead of " sulphate of soda. " The older 

 names are, however, still used in commerce and in 

 pharmacy. 



Strength of acids. The fact that oil of vitriol could 

 displace muriatic acid from common salt, and nitric acid from 

 nitre, was recorded by Glauber. Attention was directed at 

 first mainly to the liberated acid, but it was recognised soon 

 that a soluble substance was left behind which contained the 

 " fixed salt " (i.e., the base or alkali) of the original substance 

 in combination with the stronger vitriolic acid. Thus, 

 when oil of vitriol acted upon common salt, the residue of 

 Glauber's salt was found to be identical with sulphate of 

 soda prepared by the action of oil of vitriol on soda ; when 

 nitre was used, the residue was a "vitriolated tartar" or 



