36 



THE SOLUBLE CONSTITUENTS. 



present in milk ; thus the phosphorus and sulphur of the proteins 

 give rise to phosphoric and sulphuric acids ; and carbon dioxide 

 is also formed by the oxidation of organic carbon. The ash 

 does not represent the true mineral constituents of milk. 

 The average composition of the ash of milk is 



Lime, 

 Magnesia, . 

 Potash, . 

 Soda, 



Phosphoric acid, 

 Chlorine, . 

 Carbon dioxide, 

 Sulphuric acid, . 

 Ferric oxide, etc., 



Less = Cl, 



103-15 

 3-15 



100-00 



The amount of insoluble ash i.e., ash insoluble in hot water 

 amounts to about 0-52 per cent, of the milk ; and the soluble 

 ash to 0-23 per cent. The soluble ash consists mainly of the 

 chlorides of the alkalies, with a little carbonate and a mere trace 

 of phosphates. 



The insoluble ash is mainly composed of double phosphates 

 of the formula CaKP0 4 , the lime being partially replaced by 

 magnesia and the potash by soda ; double carbonates of the 

 formula CaNa 2 (C0 3 ) 2 also exist in traces ; these compounds are 

 insoluble in water, and this accounts for the fact that the insoluble 

 ash is always higher than the sum of the calcium and magnesium 



An ash of this composition is only formed when the milk is 

 homogeneous ; if it is curdled, by natural souring or by the 

 addition of acids, the precipitated lumps do not contain sufficient 

 alkali metals to form these compounds, and much calcium and 

 magnesium phosphates are formed ; on dissolving in water, 

 soluble alkaline phosphates go into solution, and calcium and 

 magnesium phosphates, together with varying proportions of 

 double phosphates, are left insoluble. Curdled milk gives the 

 same total proportion of ash as fresh milk, but the soluble ash is 

 higher and the insoluble ash lower. 



Phosphoric acid equal to about 8 per cent, of the ash is derived 

 from the phosphorus of the casein ; the traces of carbonic acid 

 present are not true mineral constituents of the milk. 



Deducting these, we have a considerable excess of bases over 

 acids ; in the milk these bases are combined partly with the 



