438 THE ACIDITY OF MILK, 



produce an excess of H or OH ions as the case may be. To pass 

 from an excess of H ions to an excess of OH ions, only the 

 addition of a small amount of alkali is required. But, on the 

 other hand, in the solution of phosphates the points at which an 

 excess of H or OH ions occurs are widely separated and much 

 more acid or alkali must be added to pass from one side to the 

 other. 



The phosphates thus possess a regulating influence oft the con- 

 centration of H and OH ions. In such a solution the concentra- 

 tions of the two opposed ions may be regarded as mutually 

 equivalent. The addition of acid or alkali will not alter these 

 concentrations until much acid or alkali is present. The fluid 

 will thus exhibit the characters of a neutral fluid, should small 

 amounts of acid or alkali be added to it. Important applications 

 of this regulating mechanism have been noted in the case of 

 blood-serum, lymph, and urine as well as with milk. 



During 1906 a large number of samples of milk obtained in 

 New South Wales have been examined. In the case of milk 

 bought in Sydney the reaction to litmus paper has been acid. 

 This has been the case with every sample obtained, even with 

 those that have been brought from the dairy to the laboratory in 

 the morning. On first notice this acidity was ascribed to lactic 

 acid. On neutralisation of this acidity with ~ NaOH the 

 behaviour was no longer that of fresh milk.* A little investiga- 

 tion showed that the acidity was not due to lactic acid present 

 in the milk. Attention was then paid to milk immediately after 

 removal from the cow, and my friend, Mr. J. M. Petrie, D.Sc, 

 made a number of determinations at Singleton, for which I am 

 greatly indebted to him. 



To determine the capacity of milk to combine with alkali the 

 milk has been titrated with —$ NaOH, using phenolphthalein as 



* See Moseley & Chapman, Proc. Linn. Soc. New South Wales, Vol. xxxi. 

 p.568, 1906. 



