CARBONIC ACID GAS. 69 



117. Although carbonic acid, being a gas, is not 

 perceptibly sour to the taste, like the strong acids 

 just mentioned, it nevertheless combines with bases, 

 and in so doing neutralizes, or at least weakens to a 

 considerable extent, their caustic properties ; but as 

 it has a far less powerful attraction for bases than 

 most other acids have, it is very easily expelled from 

 compounds containing it, by the action of another 

 acid, which seizes upon the base, and sets the car- 

 bonic acid gas at liberty. 



118. We have already spoken of the conversion of 

 chalk into lime by heat, in which process the chalk is 

 decomposed, certain matters being expelled or driven 

 off, and the lime left caustic. Chalk or limestone is 

 a compound of carbonic acid gas and caustic lime, 

 and is called a carbonate of lime (233). 



119. The carbonic acid is combined with the lime 

 by so weak an attraction, that heat alone is sufficient to 

 expel the acid, together with a small quantity of water 

 which the chalk always contains. If a piece of chalk 

 is put into some vinegar, or indeed into any sour 

 liquid, the chalk will be decomposed, and the car- 

 bonic acid will bubble through the fluid, until the vine- 

 gar or other acid is fully combined with lime, and its 

 acid powers entirely neutralized. The same will 

 happen with any other carbonate. 



120. In consequence of this gas being considerably 

 heavier than common air (85), it frequently collects 

 in caverns, cellars, and other similar situations, and 

 often occasions fatal accidents, suffocating those who 



