bb ACIDS AND BASES. 



takes place in the organs of plants : in the latter 

 case, carbonic acid gas in the air is decomposed, and 

 the carbon which it contained is appropriated by the 

 plant; whilst in the lungs of an animal, carbonic 

 acid is formed ; for the oxygen of the air is found, 

 on being expelled from the lungs, to have combined 

 with carbon, and become converted into carbonic 

 acid gas. In fact, the process of breathing is very 

 similar to that of combustion, the same results being 

 produced in either case (605). 



108. Carbonic acid gas is invisible and transparent 

 like common air, slightly soluble in water, and re- 

 maj*kable for being much heavier than air, and for 

 extinguishing flame and destroying animal life ; it is 

 called an acid, although it certainly is not acid or 

 sour to the taste, like vinegar and the other common 

 acids we are acquainted with; it will, therefore, be 

 proper, before proceeding any further, to explain 

 why it is called an acid, and in fact what is meant 

 by that term. 



109. There exists in Nature a numerous class of 

 substances which are called bases; amongst which are 

 potash, soda, and lime, &c. Now an acid is a sub- 

 stance that has a strong affinity for these bases, and 

 which in combining with one of them forms a neutral 

 compound, possessing none of the properties of either. 



110. Some acids are exceedingly sour, and very 

 corrosive substances, like oil of vitriol, and aqua- 

 fortis; but when poured upon a base, such as potash 

 or soda, they combine with it directly, and both the 



