16 INTRODUCTORY — THE CONSTITUENTS OF MILK. 



to crystallise. Tte product is purified by re-solution, treatment 

 with animal charcoal and re-crystallisation. In countries where 

 alcohol used for manufacturing purposes is free from duty the 

 sugar is precipitated from solution by this means instead of 

 being crystallised from water. 



On a small scale it is best to precipitate the protein from 

 milk or whey by as small a quantity of acid mercuric nitrate 

 (p. 90) as possible. The clear filtrate is neutralised with dilute 

 caustic soda solution till a very faint tinge is given with phenol- 

 phthalein ; it is filtered from the precipitate thus produced, 

 which consists of mercury salts. Sulphuretted hydrogen is 

 passed through the clear solution to remove the mercuric oxide 

 dissolved by the sugar, and, after filtration from mercuric sul- 

 phide, the sulphuretted hydrogen is expelled by boiling. On 

 evaporating the solution, milk-sugar crystallises out ; crystallisa- 

 tion may be hastened by vigorously stirring the concentrated 

 solution while it is being rapidly cooled. 



Glucose and Galactose. — These are two isomeric sugars of 

 the monose type. Both are aldoses or aldehydrols, and have 

 been obtained in three modifications (a- and /3-modifications 

 and a stable equilibrium form). 



Their constitution is given by E. Fischer as 



Oliicose. Galactose. 



COH COH 



I I 



H-C-OH H-C-OH 



I I 



OH-C-H OH-C-H 



I I 



H-C-OH OH-C-H 



I I 



H-C-OH H-C-OH 



i ! 



CHjOH CH2OH 



Wohl and List have confirmed the constitution of galactose. 



They are thus isomeric sugars difiering only in the third 

 asymmetric carbon atom from the aldehyde group. 



_ It is not known whether these sugars on solution in water 

 give a constant rotation for a short time as in the case of milk- 

 sugar. Their specific rotatory powers [a]n are 



Glucose. Galactose. 



Equilibrium form, . . 52 '7^ 80-3° 



a-iuodificatiun, . . , 105° 120° 



Bi-rotation ratio, ... 2 1-5 



Both sugars give, on treatment with phenylhydrazine acetate 

 nearly insoluble osazones. Phenylglucosazone crystallises from 

 dilute alcohol in fine yellow needles melting at 204° to 205° C. 



