42 PHYSIOLOGY 



silica. The action of water charged with carbon dioxide on a silicate 

 is to cause its gradual decomposition with the formation of carbonate 

 and silica. Both these products, being insoluble, are deposited as 

 part of the earth's crust, the silica in the form of sandstone, the carbo- 

 nate as chalk or limestone. The carbon dioxide is being constantly 

 removed by water from the atmosphere and being locked up in this 

 way in the earth's crust, the process of separation of calcium carbonate 

 being aided to a marked extent by the agency of living organisms 

 themselves. The whole of the extensive deposits of limestone and 

 chalk have been separated from the sea -water by the action 

 of living organisms. With the cooling of the earth's crust which 

 is supposed to be going on, the discharge of carbon dioxide by vol- 

 canoes must get less and less, so that one can conceive a time when 

 the whole of the carbon dioxide will be bound up 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 respiration. 



Like the three elements we have already considered, nitrogen is 

 also derived directly or indirectly from the surrounding atmosphere. 

 In consequence 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 

 combined 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 



