MILK-SUGAR. 9 



urea and other bases, an odoriferous principle, and a colouring- 

 matter ; these two latter occur in very small amount, and are of 

 unknown composition. 



Colour. The colour of milk is nearly white, due not to the 

 presence of the colouring-matter just mentioned, which accu- 

 mulates in the fat, but to the interference with the passage of 

 light by the casein in pseudo-solution. When milk is viewed 

 in thin layers, especially if the bulk of the fat has been removed, 

 it has a bluish tint : the bluish tint can hardly be called a colour ; 

 it partakes more of the nature of a fluorescence, and the trans- 

 mitted light is polarised to a slight degree. 



The fat globules, being very much lighter than the medium in 

 which they are suspended and being of sufficient mass to over- 

 come the viscosity of the fluid, have a tendency to rise and form 

 a layer of cream on the surface of the milk when left to rest. 



Reaction. Milk has always, when fresh, an amphoteric re- 

 action i.e., it turns blue litmus paper slightly red and red litmus 

 slightly blue. A similar reaction is possessed by certain phos- 

 phate solutions, and it is to the presence of such in milk that this 

 reaction is due. The true explanation is that the acidity of milk 

 is due to acid phosphates, and the strength of the e acid salts 

 is of the same order as the strength of the acid of litmus. When 

 blue litmus is used for testing, a substance more alkaline than 

 the milk is introduced and equilibrium is set up ; alkali passes 

 from the litmus to the milk, and consequently the blue litmus is 

 reddened. 



When red litmus is used an acid substance in introduced, and 

 for the attainment of equilibrium alkali must pass from the milk 

 to the litmus, thereby turning it slightly blue. 



This reaction has acquired a false importance, owing to the 

 erroneous idea that neutrality as measured by the action of 

 litmus is chemical neutrality ; with the recognition of the fallacy 

 of this idea the importance of the amphoteric reaction vanishes. 



