LACTOSE. 313 



Five different modifications of milk sugar have been de- 

 scribed one hydrated crystalline form, Cj2H2.20n4-H.20, three 

 anhydrous crystalline, and one anhydrous amorphous form. 



The hydrated substance is the most important. It forms 

 large transparent rhombic or monoclinic crystals, which 

 possess well-marked cleavage. Its specific gravity is 1-534. 

 It possesses a faint sweet taste. The crystals are stable at 

 100, but at about 130 they begin to lose water, and decompose 

 at 180 with partial charring. 



Milk sugar dissolves in 5'87 parts of water at 10 or in 2*5 

 parts at 100. The solution saturated at 10 contains 14-5% of 

 sugar and has a specific gravity of 1-055. By spontaneous 

 evaporation the solution becomes supersaturated and does not 

 deposit crystals until it contains over 21 % of sugar. By cool- 

 ing hot saturated solutions down to ordinary temperatures in 

 closed vessels no crystallization occurs and a highly super- 

 saturated solution is obtained. The crystals are insoluble in 

 alcohol or ether, but very soluble in hot acetic acid. 



Lactose resembles glucose in possessing reducing properties, 

 as shown by its action upon ammoniacal silver solution in 

 the cold and upon alkaline copper solutions on heating. This 

 is in consequence of its containing an aldehyde group, and on 

 hydrolysis either by dilute acids or by an enzyme known as 

 lactase it yields glucose and galactose. 



According to E. Fischer* the constitution of milk sugar is 

 expressed by the formula 



/OCH.2 

 CH 2 OH.(CH.OH) 4 .CH< 



\OCH- (CH.OH)aCHO, 



so that on hydrolysis there results 



CH 2 OH(CH.OH) 4 CHO and CH 2 OH(CH.OH) 4 .CHO, 



i.e., the two isomeric aldoses glucose and galactose. 



Milk sugar does not readily undergo alcoholic fermentation, 

 but by the action of certain yeasts, aided perhaps by the 

 hydrolysing enzyme lactase, it can be induced. 



It is much more prone to undergo the lactic fermentation. 

 This is brought about by micro-organisms, which are always 



* Ber. 21, 2631 ; Jour. Chem. Soc. 1888, abst. 1267. 



