112 NITRATE OF LIME. 



are all compounds of lime and sulplmric acid, and pre- 

 cisely similar in composition. Common plaster of 

 Paris is dry sulphate of lime, or gypsum, deprived by 

 heat of the water which it naturally always contains, 

 and, when mixed with a small quantity of water, it re- 

 combines with it, and becomes a hard, dry, solid sub- 

 stance. 



238. The burning of gypsum to make plaster of 

 Paris, is quite different from the burning of chalk to 

 make quicklime : in the former case the native sul- 

 phate of lime, which contains water, is heated and the 

 water thus expelled ; whilst in the burning of lime not 

 only is the water expelled which the chalk usually con- 

 tains, but also the acid itself, with which the lime was 

 combined. 



239. Sulphate of lime is dissolved in small quantity 

 by water, and accordingly we find it almost always 

 present in spring-water, which derives it from the soil 

 through which it rises ; it is the presence of this sub- 

 stance in spring-water which gives it that hardness 

 which distinguishes it from common river-water, and 

 renders it less pleasant to use than soft water ; in fact, 

 one of the great distinctions found between hard and 

 soft water is the presence or absence of sulphate, or 

 carbonate of lime, which, though contained in but 

 small quantity, greatly influences its usefulness for 

 domestic purposes (232). 



240. Nitrate of lime is a white salt, easily soluble 

 in water, and readily formed by acting on lime or its 

 carbonate by nitric acid. It is also formed whenever 



