110 CARBONATE OF LIME. 



reason, buildings of freestone or limestone are much 

 less durable than those which are built of granite, a 

 stone which does not contain carbonate of lime, and 

 is, therefore, not thus acted on by rain-water. 



231. In this kind of action, no chemical change is 

 effected: the carbonate of lime is merely dissolved by 

 water holding in solution carbonic acid ; hence when, 

 either from heat or any other cause, the carbonic acid 

 is expelled from the water, it is no longer able to hold 

 the carbonate of lime in solution, which is therefore 

 deposited again unaltered. 



232. Spring-water, for example, which often holds 

 carbonate of lime thus dissolved, becomes milky or 

 turbid when boiled, and not unfrequently deposits fur 

 in the inside of boilers and kettles ; such water is of 

 course not fit for domestic uses, cooking, &c.; but by 

 simple boiling it is at once rendered far better, the 

 chalk being then separated. 



233. The uses of carbonate of lime are very nume- 

 rous : one of the most important is for burning into 

 lime. This is a very curious process, for heat gene- 

 rally increases the attraction which two different sub- 

 stances have for each other ; yet in this and some 

 other cases it rather diminishes it, and the strongest 

 heat is unable to drive off carbonic acid from its com- 

 bination with potash or soda, though lime, which 

 parts with it so readily when heated, is able when cold 

 to take it from either of the alkalies. 



234. When quicklime is mixed with vegetable or 

 animal substances, it renders the process of putrefac- 



