NlTllOGEN. 7 



rarely save in substances which owe their origin to animal or 

 vegetable life. Indeed of purely mineral substances containing 

 it we know of very few, if any. The deposits of nitrates, e.g., 

 of sodium and potassium nitrates, which are found in certain 

 hot climates and which are largely used as sources of combined 

 nitrogen, have almost certainly been formed by the same agen- 

 cies which produce nitrates in all fertile soils bacteria, and 

 probably from the same sources organic nitrogenous bodies. 

 Unlike other elements, nitrogen appears to occur only on the 

 outermost parts of our globe, i.e., either in the atmosphere or, 

 if underground, within a very short distance of the surface of 

 the earth, the only noteworthy exception to this being the 

 occurrence of coal and carboniferous shale, &c., which usually 

 contain about 1 % combined nitrogen. It must be remem- 

 bered that these deposits are of vegetable origin and were 

 formed at the surface. The properties of free nitrogen are 

 well known ; it is a colourless, odourless gas, possessing little 

 chemical activity, taking part in very few of the changes 

 occurring in the atmosphere. Indeed its most remarkable 

 characteristic is its general chemical inertness. It accom- 

 panies oxygen in all the multitudinous processes in which the 

 latter takes such an active part and in most instances escapes 

 unchanged. Only by very extreme means can it be caused to 

 combine with other substances and usually heat is absorbed by 

 the act of union. 



The compounds of nitrogen, unlike the element itself, are 

 extremely active chemically and many of them are of great 

 importance. 



All organisms, whether plant or animal, require nitrogenous 

 compounds to build up their tissues. Compounds containing 

 nitrogen therefore are essential ingredients in the food supply 

 of both plants and animals. Many most powerful medicines 

 and poisons contain nitrogen, e.g., all the alkaloids brucine 

 (C. 23 H 2(; NA), strychnine (C. 21 H. 22 N. 2 O) , quinine (C. 20 H. 24 N 2 O 2 ), 

 &c. prussic acid (HCN), &c. Many nitrogen compounds are 

 extremely unstable, i.e., they readily split up into simpler 

 compounds, the nitrogen being generally set free, thus they 

 are often violently explosive, e.g., nitro-glycerine, C 8 H 5 (NO s ) ;i , 

 gun-cotton, C (i H 7 0. 2 (NO ;! ) ;! , &c., &c. 



