CHAPTER III. 



THE SOLUBLE CONSTITUENTS. 



Milk-Sugar, Lactose (Lacton or Lacto-biose), C 12 H 22 O n . OH 2 . 

 Properties. This eugar 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 2 

 CH 8 OH . 4(CHOH)CH 



OCH3(CHOH)COH. 



The aldehyde group of the galactose has been eliminated in 

 milk-sugar, while that of the glucose remains. This is shown 

 by the reactions of several derivatives of milk-sugar ; by heating 

 milk-sugar with phenylhydrazine and acetic acid, phenyl-lactos- 

 azone is formed, which yields an osone on treatment with strong 

 hydrochloric acid ; this, by boiling with hydrochloric acid, yields 

 a mixture of galactose and glucosone. By treatment again with 

 phenylhydrazine, the glucosone forms phenylglucosazone almost 

 immediately, and, on warming, phenylgalactosazone is precipi- 

 tated. A clear demonstration is thus afforded that the aldehyde 

 group of the glucose only remains. 



By oxidation with bromine, lactobionic acid is formed, which 

 is hydrolysed by acids to gluconic acid and galactose ; again 

 showing that the galactose group is modified. 



The reactions of milk-sugar, which are all displayed in solu- 

 tion, are those of an aldehyde, but from its formation of stable 

 hydrated compounds it appears more correct to regard it as an 

 aldehydrol. 



Modifications. Milk-sugar exists in several modifications 

 which are distinguished from each other chiefly by their beha- 

 viour towards polarised light. 



The best known modification is the hydrated a-milk-sugar, 



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