CHEMICAL PROPERTIES OF MILK-SUGAR. 31 



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

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

 from 1 -09 to 1 -02, such sugar probably consists of the equilibrium 

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

 specimen having a bi-rotation ratio of 1-03 had a density of 



15*5 

 1-585 at YF^O an d dissolved in water with a slight evolution of 



heat. 



There is some evidence that the hydrated ^-modification dis- 

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

 fication, as the mixtures obtained by precipitation with alcohol 

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

 The solubility appears to be greater. 



By the addition of ammonia, the change which the a- and /?- 

 modifications 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 

 often have the face at the end of the wedge greatly prolonged. 

 The /5-modification crystallises in needles. 



The taste of the a-modification is , not sweet, and from its 

 comparative insolubility it appears to be gritty. In solution 

 milk-sugar has a sweet taste of about a quarter the sweetness 

 of cane-sugar. 



As already stated, on heating to 170 C. it turns brown, and 

 lacto -caramel is formed ; a similar change takes place by heating 

 an aqueous solution to 100 C. for some hours ; the presence of 

 small amounts of alkali greatly increases the browning of the 

 solution, the rotatory power being greatly diminished. 



Chemical Properties. Milk-sugar, in common with other 

 aldoses and ketoses, reduces alkaline solutions of copper, silver, 

 and mercury, forming cuprous oxide, and metallic silver and 

 mercury respectively. On this fact the well-known Fehling's 

 test for sugar is based. The amount of reduction is constant for 

 fixed amounts of milk-sugar under the same conditions, and is 

 nearly proportional to the amount of milk-sugar. Each sugar 

 shows a definite amount of reduction in the same way, and a 

 valuable method for distinguishing them is thus available. The 

 difference between reduction by various sugars is not due to any 

 difference in the reaction with the metallic salt, but depends on 

 their relative stability towards alkalies. 



