62 A TEXTBOOK OF PHYSIOLOGY 



it becomes inverted; lactose yields distinctive crystalline rosettes, 

 which are better obtained if a little free acetic acid be also added to 

 the sugar solution; maltose yields an osazone, so soluble that it does 

 not appear until the solution is cooled. 



7. Molisch's test. All carbohydrates yield with a-naphthol and 

 sulphuric acid a purple colour. The greenish colour which is also 

 present forms no part of the test. The test is very sensitive; carbo- 

 hydrates linked to proteins yield it. 



8. The Fermentation test. This is a very useful test for aiding in 

 distinguishing the different sugars. The sugar solution is introduced 

 with a small piece of brewer's yeast into a Southall's urometer (see 

 p: 458), and placed in an incubator at 37 C. The production 

 of carbon dioxide gas indicates fermentation. A control tube of 

 yeast and water should yield no gas. 



9. The Polarimetric test. This consists in determining the rotation 

 power of a solution of known strength of the sugar. In the case of 

 disaccharides the rotatory power is determined before and after 

 inversion with acid. Each disaccharide behaves differently on hydro- 

 lysis (see table, p. 64). 



Dextrose (also termed glucose and grape-sugar) occurs abundantly 

 as such in the grape, in other sweet fruits, seeds and roots, and honey ; 

 it is more often found in conjunction with levulose. It is formed by 

 boiling starches and dextrins with dilute sulphuric acid, the syrupy 

 sugar thus formed being largely employed in beer, jam, and sweet 

 making. Occasionally arsenical impurities in the common sulphuric 

 acid (oil of vitriol) used in this process have led to cases of arsenical 

 poisoning. It is also produced from starch by hydrolytic changes in 

 the alimentary tract. Dextrose is the sugar found in the blood and 

 muscles (1 per 1,000 parts), being an important source of energ3 r to the 

 latter. Under normal conditions only the merest trace occurs in the 

 urine. In the body it is converted into and stored as animal starch 

 or glycogen. The specific power of rotation of glucose is +52-6. 



Levulose (fructose) occurs widely in conjunction with dextrose in 

 fruits, and also in honey. It is generally obtained as the result of 

 the splitting up of cane-sugar. It may occur in the urine. It is 

 readily soluble in water, but not in cold alcohol. Its levorotatory 

 power is about 93. It gives the usual reducing tests, and ferments 

 readily with yeast. Its osazone is similar to that of dextrose, the 

 melting-point being 204 C. The characteristic test for levulose is 

 known as Seliwanoff's test. When one part of HC1 in two parts of 

 water, in which a few crystals of resorciii have been dissolved, is 

 added to levulose, a deep cherry-red colour results. 



Galactose is obtained by the splitting of lactose (milk-sugar). It 

 is also obtained when certain lipoids (galactosides) are split by weak 

 mineral acids. It is less soluble in water than dextrose. It gives the 

 reducing tests. Its osazone melts at 196 to 197 C. It turns the 

 plane of polarized light to the right, its rotatory power being +81. 

 With yeast it ferments but slowly. Upon oxidation with nitric acid 



