CARBOHYDRATES 47 



There are other examples of the existence of the a and 

 modification of glucose. For example, a-methyl-glucoside and 

 /3-methyl-glucoside (specific rotatory powers, +157 and 33, 

 respectively) are both known, as well as several other similar 

 glucoside arrangements. 



Mannose. This sugar does not occur as such in plants; but 

 complex compounds which yield d-mannose when hydrolyzed, 

 known as " mannosans," are found in a number of tropical plant 

 forms. The mannose which is obtained from these by hydrolysis 

 is very similar to glucose in its properties, forms the same osazones 

 as do glucose and fructose, exhibits mutarotation, etc. Mannose 

 may also be obtained by oxidizing mannitol, a hexatomic alcohol, 

 known as " mannite," which occurs in many plants, especially in 

 the manna-ash (Fraxinus ornus), the dried sap from which is 

 known as " manna." 



Galactose occurs in the animal kingdom as one of the constit- 

 uents of lactose, or milk-sugar. It is also one of the constituents 

 of raffinose, a trisaccharide sugar found in plants, and occurs 

 as " galactans " in many gums and sea-weeds. The d-galactose, 

 obtained by the hydrolysis of any of these compounds, is a faintly 

 sweet substance which resembles glucose in many of its properties; 

 having one characteristic difference, however, in that it forms 

 mucic acid instead of saccharic acid when oxidized by concen- 

 trated nitric acid. These oxidation products are very different 

 in their physical properties and this difference serves to dis- 

 tinguish between the two sugars from which they are derived. 



Fructose (levulose, honey sugar, or " diabetic " sugar) occurs 

 along with glucose in the juices of many fruits, etc. It is a con- 

 stituent of sucrose, of raffinose, and of the polysaccharide inulin, 

 from which it may be obtained by hydrolysis. It is a ketose sugar, 

 reduces Fehling's solution, forms the same osazone as glucose, and 

 is easily fermentable by yeast. Its sweetness is slightly greater 

 than that of ordinary cane sugar, d-fructose (the ordinary form) 

 is easily soluble in water, and is strongly levorotatory, its specific 

 rotatory power at 20 C. being 92.5; it is unique in the very 

 large effect which is produced in its rotatory power by increasing 

 the temperature of the solution; at 82 its specific rotatory power 

 is reduced to 52.7, exactly equal to but in the opposite direction 

 of the effect of glucose; hence, invert sugar, which is a mixture of 

 an equal number of molecules of glucose and fructose, and which 



