MILK-SUGAR. 9 



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- 

 <ome 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 duo. The true explanation is that the acidity of milk 

 is dae to mono- and di-acid phosphates, and the strength of 

 these 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 ])asses from the litmus to the milk, ami conse- 

 <|uentl3' the blue litmus is reddened. 



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

 for the attainment of equilibrium alkali must pass from the 

 milk to the litmus, thereby turninu 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. 



The Chemicwl Properties of the Constituents of Milk. 



Milk-Sugar, Lactose (Lacion or Lavto-hiosc). CioH,.0,i UHo. 

 — Properties. — This sugar is found in the milk of the cow and 

 probably in that of most other mammals. It is a hexa-biose, 

 and belongs to the class of aldehydes (aldoses), or rather alde- 

 hydrols. It has the constitution of a galactose-glucoside, and 

 on hydrolysis by acids yields a mixture of galactose and glucose. 

 Fischer assigns the following constitution to it : — 



OCH., 

 CH.OH . 4(CH0H)CH 



OCH.?(CHOH)COH. 



