16 ELEMENTARY AGRICULTUKAL CHEMlSTUr 



diamond is crystalline, transparent, and about three and a half 

 times as heavy as an equal bulk of water ; graphite is crystalline, 

 opaque, and about two and a half times as heavy as water ; 

 while amorphous carbon is, of course, non-crystalline, opaque, 

 and (when its pores are filled with water) about one and a half 

 times as heavy as watei* ' The black colour which is produced 

 when animal or vegetable wabstances are strongly heated with- 

 out access of air ("charring") is due to the separation of free 

 carbon from the various carbonaceous compounds present ; 

 indeed, the charring of a substance when it is heated can 

 usually be taken as an indication that it contains organic or 

 carbonaceous matter. Chemically, carbon is remarkable for 

 its power of uniting in a vast number of different proportions 

 with hydrogen and with hydrogen and oxygen. Of such com- 

 pounds thousands are known, and their study comprises that 

 branch of science known as organic chemistry, which, though 

 of comparatively recent origin, has already attained vast 

 dimensions. 



NitrOg"en is much less abundant in nature than the elements 

 already described. A peculiarity of its occurrence is the 

 fact that it appears to be present only in the outermost portion 

 of the earth, the greater portion being free in the air. No 

 true minerals containing it, except those which owe their 

 origin directly to plant or animal life, e.g., coal and Chili 

 saltpetre, are known. All living matter, however, contains it 

 as an essential constituent. 



It can readily be obtained from its compound with hydrogen, 

 ammonia, by removing the hydrogen either by means of oxygen 

 or chlorine. Its properties are chiefly of a negative character, 

 for it shows little tendency to combine with other elements. 

 Although in the free state it is so inert, the nitrogen compounds 

 are as a rule, possessed of great chemical activity, and many of 

 them are very important substances. Many powerful drugs 

 and poisons contain nitrogen, e.g., quinine, CjoHg^lSTgO^j strych- 

 nine, CjjHjjNgO, prussic acid, HCN ; while most explosives, 



