10 Agricultural Chemistry. 



tilizers to produce the carbon compounds of the plant, because 

 the carbon dioxide of the air is the source for crop production. 

 It is estimated that there are about thirty tons of carbon dioxide 

 in the air over every acre of the earth's surface. 



This element occurs in three distinct forms: (1) as the 

 diamond, (2) as graphite and (3) as charcoal, lamp black, etc. 

 The diamond is crystalline and transparent ; graphite is crystal- 

 line but opaque ; while lamp black and charcoal are non-crystal- 

 line. The black carbon which is produced when animal or vege- 

 table substances are strongly heated without access of air (char- 

 ring) is due to the separation of free carbon from the various car- 

 bonaceous compounds present. 



Nitrogen (N) is much less abundant in nature than the ele- 

 ments already described. A peculiarity of its occurrence is 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 con- 

 taining it are known except those which owe their origin directly 

 to plant or animal life, as coal, and Chili salt-petre. All living 

 matter, however, contains it as an essential constituent. In its 

 free state it is a colorless, odorless gas, showing little tendency to 

 combine with other elements. It constitutes about seventy-nine 

 per cent of the atmosphere and over each acre of land there is 

 consequently about thirty thousand tons. 



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

 as a rule, possess great chemical activity and many are very im- 

 portant substances. Some powerful drugs and poisons as quin- 

 ine, strychnine, and prussic acid contain nitrogen, while most ex- 

 plosives, as nitro-glycerine and gun cotton are also nitrogen com- 

 pounds. It is an absolutely essential ingredient in the food of 

 both animals and plants. It must be supplied to animals in com- 

 pounds in which it is combined with carbon, hydrogen, oxygen, 

 and certain other elements and which are known as proteins, 

 while plants acquire it generally from nitrates, which are simple 

 compounds of oxygen, nitrogen, and some base, as calcium, so- 

 idum, and potassium. Only under very special conditions can 



