10 INTEODDCTOEY — THE CONSTITUENTS OF MILK. 



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 phenylglacosazone almost 

 immediately, and, on warming, phenylgalactosazone is precipi- 

 tated. A clear demonstration is thus afEorded 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. 



Modifleations. — Milk-sugar exists in several modifications 

 which are distinguished from each other chiefly by theix beha- 

 viour towards polarised light. 



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

 usually known as crystallised milk-sugar ; this is the form in 

 which it crystallises from water. The i^-modification exhibits, 

 multi-rotation — i.e., when dissolved in water it has a much higher 

 specific rotation than that which it attains after a lapse of time. 

 The author has found that when more milk-sugar than will 

 immediately dissolve is shaken with water it causes a lowering 

 of the temperature of the solution by about 0"55° C. By shaking 

 up the finely powdered sugar with water, a solution is obtained 

 containing about 7 '5 grammes per 100 c.c. at 15° C, the quantity 

 dissolved increasing roughly about 01 gramme per 100 c.c. 

 for each degree above 15° C. No thermal change was detected 

 in this solution by a thermometer reading to 0'01° C, but the 

 temperature rose steadily till it attained that of the surrounding 

 atmosphere, which was kept constant ; the rate of rise was 

 identical with that of a previously prepared solution of milk- 

 sugar of the same strength, which had been cooled to the same 

 temperature. Brown and Pickering have, however, shown that 

 a slight thermal change takes place with change of rotatory 

 power (+0-19 calorie per gramme). No change in density or 

 . molecular weight indicated by freezing point determination was 

 observed on keeping solutions of milk-sugar, though the specific 

 rotation varied very widely. 



It is usually stated that a freshly prepared solution of a-milk 

 sugar contains 14-55 per cent, at 10° C. ; while by long standing 



