360 



SCIENTIFIC RECREATIONS. 



Oxygen and nitrogen combine under the influence of electricity, as 

 shown by Cavendish, who passed a current through an atmospheric mixture 

 of oxygen and nitrogen, in a tube terminating in a solution of potash, lime, 

 and soda. Every time the spark passed, the volume of gas diminished, and 

 nitric acid was formed, as it is in thunderstorms, when it does not remain free, 

 but unites with ammonia, and forms a highly useful salt, which promotes 

 vegetable growth. Here is another instance of the usefulness of thunder- 

 storms, and of the grand provisions of nature for our benefit. Nitric acid is 

 obtained by distilling nitre with sulphuric acid. The liquid is, when pure, 



Fig- 35 1 - Apparatus for obtaining nitrogen by using metal to absorb the 

 oxygen of the air. 



colourless, and is a powerful oxidizer. It dissolves most metals, and destroys 

 vegetable and animal substances. By an addition of a little sulphuric acid 

 the water is taken from the nitric acid, and a very powerful form of it is 

 the result. The acid is of great use in medicine, and as an application to 

 bites of rabid animals or serpents. It converts cotton waste into " gun- 

 cotton " by a very simple process of steeping, washing, and pressing. From 

 the hydraulic press it comes in discs like " quoits," which will burn harm- 

 lessly and smoulder away, but if detonated they explode with great violence. 

 As a rule, when damp, it is not dangerous, but it can be fired even when 

 wet. It will explode at a less temperature than gunpowder, and, moreover, 



