NITROGEN. 167 



The only explanation that can be offered is the fact that argon has scarcely 

 any chemical affinity for other elements, and consequently its presence was not 

 revealed by any of the ordinary reactions used in air analysis. In fact, the 

 total of argon present had invariably been reported as nitrogen up to the time 

 of the discovery of the new element. 



Helium is another gaseous element discovered in 1895. It occurs absorbed 

 in a number of rare minerals from which it is expelled by heating. It is also 

 a constituent of the gases which are disengaged from certain spring waters, 

 and, in very small quantities, is a constituent of atmospheric air. Both argon 

 and helium are very inert. Helium has an atomic weight of about 4, while 

 that of argon is 40. One volume of helium is contained in 245,000 volumes of 

 air. 



Compounds of nitrogen. Nitrogen has very little tendency to 

 combine directly with other elements, bat it is an easy matter to 

 obtain compounds of nitrogen. These, however, are all obtained in 

 indirect ways, being either furnished ready made by processes of nature 

 or obtained as by-products in manufacturing industries. Conversely, 

 as a result of the inactivity of nitrogen, most of its compounds are 

 more or less unstable, either at ordinary or elevated temperatures, or 

 when brought together with other substances. We have already seen 

 how easily ammonium nitrite is radically decomposed by heat, and 

 ammonium nitrate acts in the same way, as will be seen below. 



The two principal compounds of nitrogen are ammonia and nitric 

 acid, and nearly all the others with which we have to do in inorganic 

 chemistry are derived from these. The valence of nitrogen is 3 in 

 ammonia, which represents the limit of reduction, while it is 5 in 

 nitric acid, which is the limit of oxidation of nitrogen. 



Ammonia, NH 3 16.93. This compound is constantly forming 

 in nature by the decomposition of organic (chiefly animal) matter, such 

 as meat, urine, blood, etc. It is also obtained during the process of 

 destructive distillation, which is the heating of non-volatile organic 

 substances in suitable vessels to such an extent that decomposition 

 takes place, the generated volatile products being collected in re- 

 ceivers. The manufacture of illuminating gas is such a process of 

 destructive distillation ; coal is heated in retorts, and most of the 

 nitrogen contained in the coal is converted into and liberated as 

 ammonia gas, which is absorbed in water, through which the gas is 

 made to pass. This is the source of nearly all the ammonium salts 

 on the market. 



Another method of obtaining ammonia is through decomposition 

 of ammonium salts by the hydroxides of sodium, potassium, or cal- 



