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lime, which contains water, is heated and the water thusexpelled ; whilst in the burning of lime not only is thewater expelled, but also the acid itself with which thelime was combined.
168. Sulphate of lime is dissolved in small quantityby water, and accordingly we find it almost alwayspresent in spring-water, which derives it from the soilthrough which it rises ; it is the presence of this substancein spring-water which gives it that freshness which dis-tinguishes it from common river-water, and renders itpleasanter to drink than soft water; in fact, the great dis-tinction found between hard and soft water is mainlycaused by the presence or absence of sulphate of lime,which, though contained in very small quantity, greatlyinfluences its usefulness for domestic purposes.
169. There is only one other compound of lime ofmuch importance, and which has already been mentioned:this is the phosphate. It is found in small quantity inmany plants, and forms a great part of the solid matterof the bones of animals. (125.)
170. Phosphate of lime is by no means an abundantsubstance, but it exists in small quantity in a great manysubstances. Traces of phosphoric acid are found in thesoil, in almost all plants and animal matters. It neveroccurs free, or uncombined, but always in combinationwith a base, and this is very commonly lime. Thus wealways find phosphate of lime in wheat and other grains,which constitute part of the food of man ; and we findit in a very considerable quantity associated with car-