108 SULPHATES OF MAGNESIA AND IRON. 



merits, it is of importance that it should not have been over- 

 burned that is, heated above 300 F. as then it neither 

 combines readily with water, nor is easily dissolved in the 

 soil. 



4. Sulphate of magnesia, (Epsom salts,) is without colour 

 or smell, but has a peculiar disagreeable bitter taste, only a 

 little less unpleasant than that of sulphate of soda. It crystal- 

 lises in four-sided prisms, which undergo a very slight efflor- 

 escence by exposure to dry air at ordinary temperatures, and 

 dissolve in 1 J parts of water at 60. The solution produces no 

 sensible effect on animal or vegetable substances. 



The pure sulphate of magnesia of the shops consists of 



Sulphuric acid, .... 32.40 



Magnesia, .... 16.70 



Water, 50.90 



100 



The large percentage of water contained in this salt, and in 

 the crystals of sulphate of soda, as well as their extreme solu- 

 bility, are important characters of these two sulphates. 



5. Sulphate of iron, (green vitriol,) is of a pale green 

 colour, and is without smell ; but it has a peculiar styptic taste, 

 characteristic of the salts of iron. When exposed to the air for 

 a length of time, it gradually assumes a rusty appearance on 

 the surface, from the production of peroxide of iron, by the 

 action of the oxygen of the air. It dissolves in 1 J parts of 

 water at 60, giving a pale green solution, which has no imme- 

 diately sensible effect upon animal or vegetable substances 

 introduced into it, but which speedily becomes yellow by 

 exposure to the air ; deposits a yellow ochrey sediment, and 

 acquires a strongly acid taste. 



The sulphate of iron, or green vitriol, consists of 



Sulphuric acid, . . . . 31.03 



Protoxide of iron, . . . 27.19 



Water, 41.78 



100 



