INTRODUCTION . 17 



e.g., nitroglycerine, CjIT^NgO^, gun-cotton, CJIyNjO^, and 

 many others, are also nitrogen compounds. It is an essential 

 ingredient in the food of both animals and plants. To the 

 former it must be supplied in combination with carbon 

 hydrogen, oxygen and other elements in the complex com- 

 pounds known as albuminoids, while plants require it chiefly 

 in the form of nitrates. Only under very special conditions 

 can some species of plants obtain the nitrogen they require 

 from the free nitrogen of the air. As will be seen in sub- 

 sequent chapters, although plants are surrounded with air, 

 which contains about three-quarters of 'its weight of nitrogen, 

 comhined nitrogen is one of the essential and most valuable 

 constituents of manures. A large portion of the nitrogen in 

 the food consumed by men and animals is eliminated in the 

 form of urea and other compounds in the excreta. Unfortu- 

 nately these are in many cases sent down the sewers into the 

 rivers, which are thereby polluted, and finally are discharged 

 into the sea. The valuable combined nitrogen is thus wasted 

 80 far as its utilisation in agriculture is concerned. 



Sulphur is found both free and combined in nature. The 

 free element is found in volcanic districts, particularly in 

 Sicily ; while in the combined state it occurs as sulphuretted 

 hydrogen, H^S, in many mineral waters ; as sulphides of many 

 metals, e.g., of iron, as iron pyrites, FeS^, of lead, as galena, PbS, 

 of zinc, as blende, ZnS; and also as sulphates of certain 

 metals, e.g., of calcium, as gypsum or selenite, GiiS0^.2Hfi, 

 and anhydrite, CaSO^ and of barium, as harytes or heavy spar, 

 BaSO^. Calcium sulphate is very widely distributed, and 

 being soluble in water is to be found in most spfing and river 

 waters. 



Sulphur can be obtained by the partial oiddation of sul- 

 phuretted hydrogen : 



2H,S+0, = 2H,0 + 2S. 



In this way large quantities of sulphur are now obtained, the 



B 



