SULPHATES. 307 



acid. It is to be understood that this reaction between 

 water and common salt, takes place only when sulphuric 

 acid is present. The method of making the experi- 

 ment is given in the paragraph on the preparation of 

 hydrochloric acid. 



What is said 115. Sulphate of soda may be obtained 

 of its crystals? m cr y s tals, by evaporation. These crys- 

 tals, like those of many other salts, lose their combined 

 water, on exposure to the air, and become converted 

 into a white powder. This change is called efflores- 

 cence, and the salt which experiences it is called efflo- 

 rescent. In preparing the salt on a large scale, for 

 conversion into carbonate of soda, large quantities of 

 hydrochloric or muriatic acid are incidentally pro- 

 duced. 



Whatissul- 77^' SULPHATE OF BARYTA. The sul- 

 pkate of ba- phate of baryta is a white insoluble sub- 



ryta? How . / 



prepared? stance, which may be obtained, as a pre- 

 cipitate, by double decomposition of any 

 soluble baryta salt with a soluble sulphate. It is a 

 mineral of frequent occurrence, known as heavy spar. 

 It is used for the adulteration of white lead, in which 

 it may be easily detected as a residue, on dissolving 

 the white lead in dilute nitric acid. The sulphate of 

 lead is another of the few insoluble sulphates. 



777. ALUM. Ordinary alum is a double 



Describe \ 



alum, and its sulphate of alumina and potassa. Solu- 



prcparation. ^^ of the tWQ ^^ when mixed? CQm _ 



bine to form the double salt. The sulphate of alu- 

 mina required in the process may be obtained by dis- 

 solving alumina from common clay by sulphuric acid. 



