THE MINERAL SALTS n 



associated with them in some way. Some protein compounds 

 contain phosphorus as a constituent element (pp. 74, 79). 

 Apart from this, the formation of prot:ins in plants appears to 

 depend upon the presence of phosphates. In the plants calcium 

 salts react and form compounds with injurious or superfluous 

 products, e.g. oxalic acid, chlorine, etc. These find their way, 

 by processes of diffusion, towards the exterior parts of the 

 plants, where they are either deposited as insoluble salts or 

 are washed off by rain. The calcium and magnesium com- 

 pounds are probably also connected with the vital functions 

 of the organism in other ways. 



In the case of animals, the relations between the phosphates 

 and the proteins are probably not less important. But, in 

 vertebrate animals, the phosphoric acid and lime are chiefly 

 concentrated in the bones in the form of tricalcic phosphate, 

 Ca 3 (PO4) 2 . It is this substance which gives rigidity to the 

 bones. The proportion of phosphates in bones varies with 

 the kind and age of the animal, but usually, in full-grown 

 mammals, nearly half the mass of the bones consists of tricalcic 

 phosphate. 



The phosphates in the food are, therefore, to a large extent 

 retained by growing and milk-producing animals, and the 

 droppings, which go to form the farmyard manure, contain 

 relatively small amounts of that ingredient. Indeed, it was 

 the perception of this fact which led to the introduction of 

 "artificial" phosphatic manures. These were first used in the 

 form of bones, and the idea was to restore to the land the 

 phosphates which are retained by the animals to build up their 

 skeletons. 



The question then arises whether it is necessary or advisable 

 to increase the amount of phosphates and lime by direct 

 addition of such compounds to the food of growing and milk- 

 producing animals. There is very little certain information 

 on this point. A few experiments have been carried out, and 

 though the results appear to indicate that the direct addition 

 of phosphates to the food had a favourable effect upon the 

 growth of the animal, the difference was too small to be of 

 much importance. Such additions cannot be regarded as 



