MODIFICATIONS OF MILK-SUGAR. 13 



in water and taking readings at intervals ; as some of the read- 

 ings obtained kave been above, and some below, that ultimately 

 obtained, it is probable 'that the very slight differences noticed 

 were due to errors of observation. 



By precipitating less strong solutions of milk-sugar by alcohol, 

 products can be obtained which contain very nearly, if not quite, 

 the same percentage of water as the hydrated a-modification. 

 but which have a much smaller, but not constant, bi-rotation 

 ratio. These also give a constant rotation for a few minutes on 

 dissolving in water and behave as mixtures of the a'"and /5-modi- 

 fications. They have a less density than the a-modification, 

 but it is not certain whether this may not be due to the very 

 imperfect crystallisation which takes place, the products appearing 

 nearly amorphous. 



By evaporating aqueous solutions of milk-sugar on the water 



bath, an anhydrous sugar can be obtained, which has a very 



slight bi-rotation ratio, which is not constant. As this varies 



from r09 to r02, such sugar probably consists of the equihbrium 



form, mixed with a small amount of the a-modification. A 



specimen having a bi-rotation ratio of r03 had a density of 



15'5° 

 r585 at ■■ - -o and dissolved in water with a slight evolution of 



heat. ^^'^ 



There is some evidence that the hydrated /3-modification dis- 

 solves in water with a greater absorption of heat than the a-modi- 

 fiiation, as the mixtures obtained by precipitation with alcohol 

 cause a greater lowering of temperature than the a-modification. 

 The solubility ajjpt'ais to be greater. 



There exists also a ;9-modifl.cation, which is obtained in the 

 anhydrous form b)' the rapid evaporation of aqueous solutions 

 in metallic vessels. It has a specific rotatorv power of S2'7° 

 at the moment of solution. Bchmoeger states that it has a 



bi-rotation ratio of ,— , which Tanret confirms. The rate of 



I'D 



change of the a- and /J-modifications is the same for the same 

 temperature. 



By the addition of ammonia, the change \\hich the a- and fi- 

 moditications slowly undergo on solution in water becomes 

 almost instantaneous. By raising the temperature, the rate of 

 change is increased and is practically instantaneous on boiling. 



The solubility of milk-sugar in water is small compared with 

 the solubilities of other carbohydrates : owing to the tendency of 

 milk-sugar to form supersaturated solutions it is difficult to 

 determine its exact solubility, but the mother liquors from 

 which crystals have deposited usually contain about 21 per cent. 

 The a-modification crystallises in wedge-shaped forms which 



