THE INORGANIC CONSTITUENTS OF MILK. 27 



have the power of converting milk-sugar into lactic acid and alcohol. By 

 the gradual formation of free lactic acid in the process of lactic fermenta- 

 tion, the lime and alkaline salts, present in milk possessing a faint alkaline 

 reaction, are gradually changed, and the amphoteric reaction of milk 

 disappears, and the acid reaction alone remains, and gradually increases in 

 strength. With the lapse of time this takes place to such an extent that, 

 although the milk remains liquid at ordinary temperatures, a slight 

 increase in temperature, or the introduction of carbonic acid, causes 

 coagulation of the milk. Finally, the casein, even at ordinary tempera- 

 tures, is decomposed from its combination with chalk, and is precipitated 

 in the form of a white, cohesive gelatinous mass, which encloses all the 

 remaining constituents of the milk. 



8. The Inorganic Constituents of Milk (Mineral, Incombustible, or 

 Ash Constituents). The mineral salts of milk, as has already been 

 indicated, form a very important part of the milk, inasmuch as they 

 influence its properties considerably. When one carefully ignites a 

 portion of milk, a mineral residue is obtained possessing a weak 

 alkaline reaction, which, on treatment with strong acids effervesces, 

 and which, therefore, contains carbonic acid, but at the very most 

 not more than 2 per cent. This residue varies in most cases 

 between *4 and '86 per cent, and constitutes on an average '75 per 

 cent of the milk. Closer examination will reveal, in addition to 

 small quantities of carbon, compounds of the metals potassium, 

 sodium, calcium, magnesium, and iron, in combination with chlorine, 

 phosphoric acid, sulphuric acid, and carbonic acid. If it be desired 

 to make a quantitative determination of the ash, and to ascertain in 

 what combinations the above metals are present in the milk, the 

 followino; considerations must be taken into account: 



o 



(1) The carbonic acid present in the ash of the milk is formed, if 

 not entirely, yet largely, during the incineration of the organic 

 nitrogenous constituents. Carbonic acid is probably not present in 

 chemical combination in fresh milk, or if it be, it is certainly only 

 in such very small quantities that its effect on the solubility of the 

 salts of milk is only of trifling importance. On this account it 

 requires no further consideration. 



(2) For the same reasons the sulphuric acid may be excluded, as 

 it occurs, at most, only in traces, and is probably not found in milk 

 at all, and is a product of the combustion of the sulphurous nitro- 

 genous matter. As casein contains "85 per cent of phosphorus, 

 every 1 per cent of casein will yield, when burned, '0194 per cent 



