THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 39 



with bases in the earth's crust, and life, without any source of carbon must 

 become extinct. 



Hydrogen exists almost exclusively in the form of water. In this form 

 it is taken up by plants and animals, with the exception of a small proportion 

 absorbed in the form of ammonia. In this form too it is discharged by living 

 organisms. Oxygen is the only element which, in all the higher organisms at 

 any rate, is taken up in the free state. It forms one-fifth of the atmosphere 

 and, as the oxides of the various metals, a considerable fraction of the earth's 

 crust. It takes a position apart from the other food-stuffs in that its 

 presence is the essential condition for the utilisation of their potential energy. 

 In the living cells it combines with the oxidisable compounds formed by the 

 agency of the living protoplasm, with the production of carbon dioxide and 

 water, and the evolution of energy. This process is spoken of as respira- 

 tion. 



Like the three elements we have already considered, nitrogen is also 

 derived directly or indirectly from the surrounding atmosphere. In conse- 

 quence of its feeble combining power for other elements and the instability of 

 its compounds, very little nitrogen is to be found in the combined state in the 

 earth's crust, whereas it constitutes four-fifths of the atmospheric gases. 

 It can be taken up by most plants only in the form of ammonia, nitrites, 

 or nitrates. To animals these compounds are useless, and the only source 

 of nitrogen to this class is the protein which has been built up by the agency 

 of the plant cell. Since nitrogen in the free state is useless to nearly all 

 living organisms, the existence of life must depend on the amount of com- 

 bined nitrogen which is available. In view of the small tendency presented 

 by this element to enter into combination, it becomes interesting to inquire 

 into the source of the combined nitrogen which is the common capital of the 

 living kingdom. There are certain cosmic factors which result in the pro- 

 duction of combined nitrogen. The passage of electric sparks or of the 

 silent discharge through moist air leads to the production of ammonium 



nitrite. 



N 2 + 2H 2 = NH 4 N0 2 . 



Every thunderstorm, therefore, will result in the production of small quan- 

 tities of ammonium nitrite, which will be washed down with the rai 

 serve as a source of combined nitrogen to the soil. Every decaying v 

 or animal tissue serves as a source of ammonia, so that from various 

 the soil may contain nitrogen in the form of ammonia or of ammoniu 

 These forms of combined nitrogen are not, however, suitable for a 

 plants. Most moulds can assimilate ammonia as ammonium carbc 

 as ammo-acids or amines, provided that they are supplied 

 time with sugar, the oxidation of which will serve hern 

 energy. Some moulds, many of the higher p ants, and espe - 



me* which include the food-producing cereals require the 

 condition of nitrates. It is necessary 

 nitrites in the soil shall be converted into this 



