66 THE CARBOHYDRATES 



peaches and plums, and from the so-called pectic substances 

 occurring in carrots and pears ; in all these cases it is ac- 

 companied by other sugars, which may be either hexoses or 

 pentoses. Galactose also occurs in several plants belonging 

 to the Caryophyllaceae. It is also formed by the hydrolysis 

 of the trisaccharide raffinose, of the glucoside digitalin, and 

 of a glucoside occurring in the ivy. 



Preparation. 



Galactose is best prepared by boiling milk sugar for six 

 hours with four times its weight of 2 per cent sulphuric acid ; 

 the solution is then evaporated, and a few crystals of galactose 

 are added to induce crystallization. After some time the crude 

 galactose crystallizes out ; it is purified by dissolving in four- 

 fifths of its weight of water, and mixing the resulting solution 

 with twice its volume of 93 per cent alcohol ; the precipitate 

 is filtered off and dried. 



Properties. 



Galactose crystallizes in minute hexagonal crystals, which 

 melt at 168°. It is strongly dextro-rotatory, a^ = 8 3 '8°, and 

 exhibits muta-rotation ; it ferments completely, but rather 

 more slowly than glucose. 



Detection. 



1. The hexagonal form of the crystals is characteristic of 

 galactose. 



2. It gives a phenylhydrazone (m.p. 158-160") which is 

 laevo-rotatory. 



3. It reduces Fehling's solution somewhat more slowly 

 than glucose ; i c.c. Fehling's solution ^ 5"i i mg. galactose. 



4. On oxidation with nitric acid it yields mucic acid. 

 Five grams of substance are heated in a beaker with 6 c.c. of 

 nitric acid (sp. gr. 1*15) until two-thirds of the liquid have been 

 evaporated off. After twelve hours the mucic acid formed 

 will have separated, and may be washed with 10 c.c. of water. 

 If other insoluble substances, such as cellulose, etc., are present, 

 place the filter paper with the solid in a dilute solution of 

 ammonium carbonate to extract the mucic acid as ammonium 



