CHAP, viii Vitality 135 



The above elements are always present, and in proportions 

 which vary within narrow limits ; but in addition to 

 these substances there seem to be always present others 

 which, when the proteids are burnt, remain as ash in the 

 form of salts chiefly chlorides of sodium and potassium, but 

 also small quantities of calcium, magnesium, and iron, as 

 chlorides, phosphates, sulphates, and carbonates. The mole- 

 cule of a proteid must be very complex ; thus that of albumen 

 is, at its smallest, C^^, H^g^, Ngg, Ogg, S^; most probably it 

 is some multiple of this. 



The food-stuffs of plants, then, are salts, water, and car- 

 bonic acid, and a certain amount of oxygen. Of these, by 

 means of the sun's energy,'they build up complex substances 

 — carbohydrates, fats, proteids, which, with salts, water, and 

 oxygen, serve as the food of animals. The living matter, 

 the machinery by which all this is done, is, if it can be 

 classed at all, a proteid. But this only means that all 

 living matter contains the five essential elements and some 

 others which in the ash exist as salts. 



The various services which the different food-materials 

 are set to within the body will be described later, when we 

 are considering the details of the animal economy. Here 

 we shall take note of the elements that enter into the 

 construction of the food-stuffs. 



7. The Chemical Elements of Life. — There are sixty- 

 eight elements to be found in varying abundance upon the 

 earth, but by analysis of the food-stuffs and of living matter 

 itself, we find that only twelve of these occur with any con- 

 stancy in organisms. They are carbon, hydrogen, oxygen, 

 nitrogen, sulphur, phosphorus, chlorine, potassium, sodium, 

 calcium, magnesium, and iron. 



Now nine of these elements form sixty-four per cent by 

 weight of the earth's crust; while aluminium and silicon, 

 substances that are only very occasionally found in living 

 creatures, form thirty-five per cent, being the chief consti- 

 tuents of quartz and felspar, sand and clay, in short the 

 greater part of all rocks. All the other elements, three of 

 which — hydrogen, nitrogen, and phosphorus — enter into 

 life, form the remaining one per cent. 



