Carbonic Acid Gas 



turns very white and, when at rest, deposits 

 flakes of a white substance. This milky dis- 

 turbance and these white flakes did not 

 appear when the air was pure ; if they are 

 formed now it must be because the contents 

 of the bottle have changed their nature. The 

 change is caused by the dissolution of the 

 carbon in the air : in other words by the 

 formation of carbonic acid gas. Hencefor- 

 ward we can recognise this gas by its property 

 of disturbing and whitening lime-water, and 

 forming a deposit of white, powderous matter. 

 This matter contains lime and carbonic acid 

 gas in combination, being neither the one nor 

 the other, but a fresh substance called 

 carbonate of lime, or chalk. The chalk that 

 we use to write on the black-board is exactly 

 the same thing — a combination of carbonic 

 acid gas and lime. But it is not obtained by 

 the method that we have used ; that would 

 be too long and too expensive. We find it 

 ready made in the bosom of the earth, like 

 clay, sand and so many other things. 



The air in which carbon is dissolved will no 

 longer maintain combustion. This may be 

 easily shown. We will go back to our wide- 

 mouthed bottle. If it is full of pure air and 

 we introduce a bit of lighted candle hanging 



