NITRIC ACID. 87 



them to remain some time together, by pouring water 

 over the mixture : this dissolves the nitrate of lime, 

 and forms a solution, which leaves the salt in ques- 

 tion, when the water is evaporated by heat. 



167. Although nitric acid is a very strong acid, 

 and has a very powerful affinity for bases, yet there 

 are even more powerful acids, which are strong enough 

 to expel it from its combinations, and we are thus 

 able to" procure the pure acid: this is usually done by 

 what is called oil of vitriol, or sulphuric acid (179), 

 a substance consisting of sulphur, or brimstone, com- 

 bined with oxygen. When this acid is poured on a 

 nitrate, it expels the nitric acid in the form of a 

 very acrid or sour and biting vapor ; this, condensed 

 in cold water, constitutes the intensely sour liquid 

 called nitric acid, or aqua-fortis, and was before com- 

 bined with the base of the nitrate. 



168. Nitric acid acts very strongly on all organic 

 bodies, decomposing them sometimes with great vio- 

 lence, and causing their elements to enter into new 

 combinations: thus when strong nitric acid is mixed 

 with oil of turpentine, it sets it on fire. This is 

 caused by the large quantity of oxygen which nitric 

 acid contains, which enables it to burn substances, or 

 at least makes them combine rapidly with oxygen, 

 which we know is the cause of ordinary combustion. 



169. When we mix nitric acid with substances 

 which, having a strong affinity for oxygen, are able 

 to decompose the acid, we obtain some of the other 

 compounds of nitrogen and oxygen before alluded to, 



