28 THE SOLUBLE CONSTITUENTS. 



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

 which it crystallises from water. The a-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 

 dissolve immediately 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 0-1 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 

 in contact with milk-sugar, or by boiling, a saturated solution 

 containing 21*64 per cent, can be obtained. The author is unable 

 to confirm the figure for the freshly prepared solution. 



1 K trO 



The density of well formed crystals is T545 at ^ , bad 



crystals i.e., those which are strained have, however, a lower 

 density. 



The hydrated a-modification is practically insoluble in 

 alcohol, ether (in ether- saturated with water it dissolves 

 to the extent of 0-00075 gramme per 100 c.c.), chloroform, 

 benzene, and other organic solvents. It is slightly, but 

 distinctly, soluble in amyl alcohol on boiling, but is probably 

 dehydrated. 



It is unaffected by heating to 100 C., but the water of hydra - 

 tion is given off at 130 C. ; at 170 a change takes place with 

 formation of lacto-caramel, and it melts at 213-5 C. 



When dissolved in water the specific rotatory power remains 

 constant for a short period, 3 minutes at 20 C., 6 minutes at 

 15 C., and 15 minutes at 10 C. ; the rotation then gradually 

 falls. 



The following series of observations (Table VIII.) will show the 

 nature of the change in rotation : 



