CHAPTER III. 



CHEMICAL COMPOSITION OF ANIMALS AND PLANTS. 



In the two living kingdoms, organised substance is, as we 

 have already seen, constituted by three groups of bodies inti- 

 mately blended, and which Chevreul' was the first to call 

 immediate principles. It is now needful to compare with each 

 other the chemical species which enter into the composition of 

 the plant and into that of the animal. We shall glance very 

 rapidly at the immediate principles of the first category. In 

 effect, water, which constitutes in weight the largest part of 

 organised beings, mineral salts, atmospheric gases, are manifestly 

 unable to furnish to us sufficiently distinctive characteristics. 

 But* that the results of the comparison may be the more striking 

 we shall indicate first of all in bold outline what is the chemical 

 composition of plants, and what is that of animals. 



1. Chemical Composition of Plants. 



Organised vegetal tissues, when submitted to desiccation, 

 present a friable residuum, the weighi; of which is very variable. 

 In the average of terrestrial plants this residuum is from a 

 fifth to a third of the total weight ; but it rises to eight-ninths 

 if we take ripe seeds, and can descend to a tenth or a twentieth 

 in aquatic plants and certain mushrooms. ^ This residuum, 

 desiccated, ofiers always to chemical analysis, carbon, hydrogen, 

 oxygen, azote and sulphur, potassium, calcium, magnesium, iron, 

 phosphorus. Often, moreover, we find therein sodium, lithium^ 



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