8 INTRODUCTION. I. 



We thus see that while free nitrogen is of comparatively 

 little value, its compounds are of the utmost importance to all 

 living beings. Consequently the means of utilising nitro- 

 genous compounds and of preventing their waste, often lead- 

 ing to the liberation of the comparatively useless free nitrogen, 

 are matters of intense interest and great importance. By 

 some chemists it is thought that supplies of nitrogen com- 

 pounds will fail us long before the want of phosphates, potash, 

 or other fertilising substances becomes felt.* 



It is to be hoped that by means of the nitrogen-fixing bac- 

 teria which grow in nodules upon the root hairs of certain 

 leguminous plants, or in some other way, it may be found 

 possible to abstract from the atmosphere sufficient nitrogen to 

 supply the wants of both plants and animals for many cen- 

 turies. Nevertheless, the rapid exhaustion of our deposits of 

 nitrates and the enormous quantities of valuable nitrogenous 

 materials which are allowed to run to waste in the sewage of 

 our large cities, are serious matters for consideration as affect- 

 ing the future supplies of this indispensable combined nitrogen. 



Carbon is perhaps more than any of the other elements, 

 associated with the processes of life. It constitutes a large 

 proportion by weight of the solid portions of all animals and 

 plants. In the mineral kingdom it is also abundant, occurring 

 in immense quantities in carbonates, e.g., those of calcium, 

 magnesium, iron, zinc, lead, copper, &c. It also occurs in the 

 air, in the form of carbon dioxide, the amount of which, though 

 small relatively to the other constituents of air, is absolutely 

 considerable and is constantly being renewed. 



The properties of the three allotropic forms of carbon are fully 

 described in any general text book of chemistry. They are not 

 of any particular importance from our present standpoint. 

 Much more important are the numerous compounds which 

 carbon forms, especially with hydrogen and oxygen, and with 

 hydrogen, oxygen, and nitrogen. Indeed it is with these com- 

 pounds of carbon that almost all the chemistry of the nutrition 

 of animals and plants is concerned, other substances taking part 

 in vital processes (though quite essential) being small in amount. 



* Sir W. Crookes, President's address, British Association for Advancement of 

 Science, 1898. 



