PHOTO-SYNTHESIS 107 



of water of crystallization. It is soluble in water, or alcohol, 

 ferments readily, rotates the plane of polarized light to the 

 right, and reduces Fehling's solution, or alkaline solutions 

 containing copper and tartaric acid. Its properties are 

 those of an aldo-hexose. Glucose is manufactured by boiling 

 starch with dilute sulphuric acid, removing the acid with 

 lime, and concentrating the liquor. 



Fructose (laevulose, fruit sugar) is also found in plants, 

 and differs from dextrose, since it is a keto-hexose. Honey 

 consists of a mixture of glucose and fructose. In cold weather 

 the glucose separates out as crystals, leaving the fructose 

 as a liquid. Crystallization of fructose presents many 

 difficulties, but the material can now be produced com- 

 mercially in the solid form. Fructose reduces Fehling's 

 solution, and rotates the plane of polarized light to the 

 left. 



Galactose, a sugar closely resembling dextrose, is not 

 generally found in plants, although it is a common result 

 of the hydrolysis of many of the gums, where it occurs 

 in combination with one of the pentoses. It is also a 

 constituent of raffinose. Many forms of yeast do not ferment 

 galactose. 



The Di-Saccharoses, C^H^On. Maltose, the con- 

 densed product of two molecules of glucose, is contained 

 in malt, and is produced from starch during the germination 

 of barley grains. It is a product of the hydrolysis of starch, 

 intermediate between dextrin and glucose. Maltose reduces 

 Fehling's solutions both before and after hydrolysis, but is 

 only fermented after hydrolysis. 



Lactose (milk sugar) is the product of condensation 

 of galactose and glucose. It occurs in cows' milk to the 

 extent of between four or five per cent., and in human milk 

 up to eight per cent., but has not been found in plants. 

 It is made from whey, a cheese by-product, by crystalli- 

 zation, and is used largely in medicine. 



Cane Sugar (sucrose) is the best known of the sugars, 

 and is contained in sugar cane, sugar beet, and many other 

 sources. Of the sugars we have dealt with above, this is the 



