CHAPTER IV 



FOOD AS A SOURCE OF ENERGY 



WHEN we carefully analyse a series of organisms 

 by appropriate chemical methods, we find that 

 twelve chemical elements are constantly present, 

 viz., carbon, hydrogen, oyxgen, nitrogen, sulphur, Analysis 

 phosphorus, potassium, calcium, magnesium, r T~ 



T j i i o i .LI ganisms. 



iron, sodium and chlorine. Several others may 

 be present in varying quantities in special cases, 

 but these twelve elements are always to be dis- 

 tinguished, and the first four are especially prominent. 

 These elements must have been introduced from 

 without, and the building up and subsequent keeping Source of 

 in repair of the organism must involve their continued elements, 

 absorption, elaboration and incorporation. The 

 ultimate sources of all the chemical elements in the 

 organism are, directly or indirectly, three in number, 

 viz., soil, water, and air. 



One most important fact, too often neglected, 

 must be especially emphasised at the very outset, 

 viz., that no protoplasm, whether of plant or of 

 animal, is able to assimilate such substances, either in 

 their elemental condition or even when united to form 

 such compounds as occur in the inorganic world. 

 Only when they are united into the very complex 

 groups which constitute what are known as organic Nature of 

 compounds can the protoplasm actually incorporate " food -" 

 or " assimilate " them, in other words, make them 

 part of itself. But these organic substances are 

 non-existent in Nature save in association with 



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