EFFECTS OF CARBONIC ACID. 35 



is all that remains, is reduced to powder, and the grains of 

 quartz and mica, having lost the cement which united 

 them, fall apart and form silicious sand. Some granites 

 contain so little quartz and mica that they are called 

 felspar rock, and these are very liable to decay, since, 

 besides being attacked by the carbonic acid, they are 

 also so absorbent of water that the winter frosts make 

 great havoc with them, 



But it is the various limestones which suffer most from 

 carbonic acid, especially in towns, where the air contains a 

 far larger proportion of the gas than it does in the 

 country. 



All limestone, chalk, and marble, properly so called, 

 consist mainly of carbonate of lime,* which is insoluble 

 in pure water ; but when it comes in contact with carbonic 

 acid, each atom of the carbonate will take up a second 

 atom of carbonic acid, and, having done this, will melt 

 as easily as sugar or salt. This we may readily prove by 

 getting from the chemist some lime-water water in which 

 lime is dissolved. We should not know from its ap- 

 pearance that there was anything in it ; but, on our 

 blowing into it through a tube, it at once becomes milky, 

 because the carbonic acid of our breath has united itself 

 to the lime and made it into the single carbonate of lime, 

 which, being insoluble, cannot be hidden in the water. 

 If, however, we go on blowing, the water will become 

 transparent again, for the lime will take up more carbonic 

 acid, and the double carbonate will be formed, and melt 

 away, making the- water " hard." 



* A compound of lime and carbonic acid. 



