122 PROGRESS OF SCIENCE IN THE CENTURY. 



ranged. The elements cannot escape the chemist, al- 

 though their intricacy of arrangement in many cases 

 does. 



If we reduce living plants to ashes, and allow 

 nothing to escape undetected, we find a constant pres- 

 ence of twelve elements, carbon, hydrogen, oxygen, 

 nitrogen, sulphur, phosphorus, chlorine, potassium, 

 sodium, calcium, magnesium, and iron. It may be 

 indeed that all the twelve are not present in some of 

 the very simplest forms of life, where the method of 

 ash-analysis is inapplicable. But for ordinary plants 

 which can be burned, the above statement is true. 

 The twelve elements are always present. Had we 

 space, it would be interesting to take each of these 

 elements in turn, to show in what forms they exist 

 in inorganic nature, to follow them from their ab- 

 sorption by root-suckers to their known combinations 

 in plant, animal, or man, and to show how they 

 eventually come back to the so-called dead-state once 

 more. But since it is better to have one definite im- 

 pression than a hundred vague ones, let us confine 

 our attention to nitrogen. 



Circulation of Nitrogen. — As is well known, free 

 nitrogen forms about four-fifths of the atmosphere, 

 but the great bulk of this takes no part in vital proc- 

 esses. With certain notable exceptions it is only in 

 the form of compounds that nitrogen can be used by 

 living creatures. Therefore, since nitrogenous food 

 is essential both to plant and animal, the amount of 

 life upon the earth must depend on the amount of 

 fixed nitrogen available.* 



The commonest circle is the following: IN'itrogen 

 is obtained by the plant in the form of nitrates, ni- 



♦Bunge, Text-tooTc of Physiological and Pathological 

 Chemistry, trans. 1890, p. 19. 



