304 EFFECT OF CHEMICAL AGENTS [Cn. XI 



30.292 parts NaCl 



3.240 " MgCl 2 



2.638 MgS0 4 



1.605 " CaS0 4 



0.779 " KC1 



0.080 " silicic acid, calcium phosphate, and insolu- 

 Total . 38.634 ble residue, including CaC0 3 and Fe 2 3 . 



From the foregoing tables it appears that mineral substances, 

 and essentially the same mineral substances, are required by 

 all organisms. The differences in this respect between the 

 different organisms are slight. Thus while sodium is not 

 included among the necessary elements of either chlorophyl- 

 laceous plants or fungi, its occurrence in considerable quantity 

 in milk, probably associated with chlorine as common salt, indi- 

 cates that it is important for some animals. 



The important conclusion seems warranted that all organisms 

 may use as hylogenic food any sort of compound which will 

 furnish the appropriate elements, but that among animals, and 

 to a less degree among fungi, organic combinations have the 

 preference because they fulfil at the same time the thermogenic 

 function. 



b. Detailed Account of the Various Elements used as Food. 

 We may now consider the part which each element plays in the 

 growth of the body as a whole, reserving for the Fourth Part a 

 consideration of the specific role which the element plays in 

 organs of the body. We may, in general, consider first the 

 share taken by the element in the constitution of the body, then 

 the form in which the element gains access to the body, and 

 finally what general effect it has upon the growth of the 

 organism. 



Oxygen. Excepting carbon, oxygen constitutes a greater 

 part of the body, by weight, than any other element. Between 

 20% and 25% of the dry substance of the human body and 

 between 35% and 45% of that of green plants is oxygen. 

 The oxygen used as hylogenic food comes to land-animals 

 from the organic compounds and the air consumed by the 

 developing young ; the oxygen of water-animals may come 

 from their food or from the oxygen dissolved in water ; finally, 



