THE NON-LIVING WORLD 139 



the greatest agent in, and supporter of, combustion, 

 oxygen itself is incombustible in air. Various of its 

 combinations or " oxides " have been already noted.* 



Hydrogen does burn in air, though it cannot aid com- 

 bustion. It is the lightest substance known, and forms the 

 chemical standard or unit of weight, and is very widely 

 diffused. Water can be resolved f into twice as much 

 hydrogen (estimated by volume) as oxygen, and as was 

 stated in the last chapter into one part of hydrogen to 

 eight of oxygen as estimated by weight. Water may 

 be called an oxide of hydrogen, and hydrogen may be 

 regarded as an aeriform metal. 



Nitrogen differs greatly from oxygen, save that, 

 like oxygen, it is colourless. It is extremely indisposed 

 to unite with other elements, and, so far from promoting 

 combustion, it stops it, extinguishing a flame plunged 

 into it. It is remarkable also for the extreme instability 

 of the compounds of which it forms a part, such as 

 gunpowder, gun-cotton, nitre-glycerine, and iodide, sul- 

 phide and chloride of nitrogen. These constitute a 

 series of substances successively exploding with greater 

 and greater violence and readiness. Nitrogen, neverthe- 

 less, is itself incombustible in air. In conjunction with 

 oxygen it forms nitric acid, which, when united with 

 other substances, produces what are called " nitrates" 



One notable substance that is resolvable into nitrogen 

 and hydrogen, is ammonia, which, under ordinary circum- 

 stances, is a gaseous as well as alkaline \ substance. It 

 is colourless but very pungent, and dissolves in water 

 with extreme rapidity, that liquid at 50 Fahr. con- 

 densing 670 times its own volume of ammonia. 



* See ante, p. 131. f See ante, p. 132. 



J See ante, p. 134. 



