SALTS OP MAGNESIA. 117 



lime, is a very deliquescent salt; that is to say, it 

 has a very strong attraction for water, and hence, 

 when exposed to damp air, it soon becomes moist, 

 and at last we find, instead of the solid salt, a very 

 strong solution of it in water. It is to the presence 

 of a small quantity of these two salts in common sea- 

 salt that its deliquescent properties before alluded to 

 are due (204). 



253. The phosphate of magnesia, like the carbon- 

 ate, is seldom found alone, but usually mixed with 

 the phosphate of lime : thus we find it associated 

 with that substance in small quantity in many animal 

 and vegetable matters, and likewise in the soil ; like 

 the phosphate of lime, it is insoluble in water, but 

 readily soluble in dilute acids. Phosphate of mag- 

 nesia has a considerable tendency to form double 

 salts (112) ; thus a double phosphate of magnesia and 

 ammonia is very frequently found in plants and 

 animals; it is readily formed artificially by adding 

 phosphate of ammonia to a solution of sulphate of 

 magnesia; it is slightly soluble in water, and has 

 been strongly recommended as a manure. The uses 

 of magnesia are limited, and the most important pur- 

 poses to which its compounds are applied are as 

 medicines. 



