28 TEXT-BOOK OF PHYSIOLOGY. 



solutions it crystallizes in fine, silky needles, though it usually occurs 

 in the form of a syrup. 



Levulose is distinguished from dextrose by its property of turning 

 the plane of polarized light to the left ; the extent to which it does so, 

 however, varies with the temperature and concentration of the 

 solution. 



Under the influence of the yeast plant it slowly undergoes fer- 

 mentation, yielding the same products as dextrose. It also has a 

 reducing action on cupric hydroxid. 



Galactose is obtained by boiling milk-sugar (lactose) with dilute 

 sulphuric acid. In many chemic relations it resembles dextrose. 

 It is less soluble in water, however, crystallizes more easily, and has 

 a greater dextro-rotatory power. It also undergoes fermentation 

 with the yeast plant. 



3. SACCHAROSES, C 12 H 22 O n . 



Saccharose, or cane-sugar, is widely distributed throughout 

 the vegetable world, but is especially abundant in sugar-cane, sor- 

 ghum cane, sugar-beet, Indian corn, etc. It crystallizes in large 

 monoclinic prisms. It is soluble in water and in dilute alcohol. 

 Saccharose has no reducing power on cupric hydroxid, and hence its 

 presence can not be detected by Fehling's solution. It is dextro- 

 rotatory. Boiled with dilute mineral, as well as with organic acids, 

 saccharose combines with water and undergoes a change in virtue of 

 which it rotates the plane of polarized light to the left, and hence the 

 product was termed invert sugar. This latter has been shown to be a 

 mixture of equal quantities of levulose and dextrose. This inversion 

 of saccharose through hydration and decomposition is expressed in 

 the following equation : 



C 12 H 22 U + H 2 = C 6 H 12 6 + C 6 H 12 6 



Saccharose + Water = Levulose 4- Dextrose 



Invert Sugar. 



Saccharose is not directly fermentable by yeast, but through the 

 specific action of a ferment, invertin or iwuertase, secreted by the 

 yeast plant, or the inverting ferment of the small intestine, it under- 

 goes inversion, as previously stated, after which it is readily fermented, 

 yielding alcohol and carbon dioxid. 



Lactose is the form of sugar found exclusively in the milk of the 

 mammalia, from which it can be obtained in the form of hard, 

 white, rhombic prisms united with one molecule of water. It is 

 soluble in water, insoluble in alcohol and ether. It is dextro-rotatory. 

 It reduces cupric hydroxid, but to a less extent than dextrose. Dilute 

 acids decompose it into equal quantities of dextrose and galactose. 

 Lactose is not fermentable with yeast, but in the presence of the 



