64 PHYSIOLOGY 



and shaking till the smell of benzoyl chloride has disappeared, an insoluble precipitate 

 is produced of the benzoic ester of glucose. This method has been often used for 

 isolating glucose from fluids in which it occurs in minute quantities. 



Molisch's Test. On treating 0-5 c.c. of dilute glucose solution with one drop of a 

 10 per cent, alcoholic solution of a -naphthol, and then pouring 1 c.c. of concentrated 

 sulphuric acid gradually down the side of the tube, a purple ring is produced at the 

 junction of the two fluids, which on shaking spreads over the whole fluid. This reaction 

 depends on the formation of f urf urol from the glucose. 



In order to identify glucose in a normal fluid, the following tests may be applied, 

 after removing any protein which may be present : 



(1) Reduction of cupric hydrate or Fehling's solution. 



(2) Estimation of reducing power of solution. 



(3) Estimation of rotatory power of solution on polarised light. 



(4) Formation of osazone crystals with phenyl hydrazine. These crystals must 

 come down while the fluid is still hot. They must be purified and their melting-point 

 taken. A determination by combustion of their nitrogen content will give direct 

 information whether the sugar is a monosaccharide or disaccharide. 



(5) The solution is made acid and boiled for some time. It is then made up to its 

 former volume and its reducing power and effect on polarised light once more taken. 

 In the case of a disaccharide, which would be converted into monosaccharide by boiling 

 in acid solution, these two readings would be altered, whereas neither the rotatory 

 power nor the reducing power of glucose would undergo any change. 



(6) Fermentation with ordinary yeast. 



A positive result would exclude glycuronic acid. 



D-FRUCTOSE, or LEVULOSE, occurs mixed with dextrose in honey 

 and in fruit sugar. It is also, with glucose, formed by the digestion or inver- 

 sion of cane sugar. It is crystallisable with difficulty. Its watery solution 

 is Isevo-rotatory, and reduces Fehling's solution somewhat less strongly than 

 glucose, its reducing power being 92, if we take that of glucose as 100. It 

 ferments readily with yeast ; with phenyl hydrazine it gives the same osazone 

 as is formed from glucose. 



GALACTOSE is formed by the digestion or hydrolysis of milk sugar or 

 lactose. It is also obtained on hydrolysing cerebrin, a constituent of the 

 brain, with dilute mineral acids, and by the hydrolysis of certain vegetable 

 gums. It is much less soluble in water than glucose. It is dextro-rotatory 

 and shows marked bi-rotation. With ordinary yeast it ferments but ex- 

 tremely slowly. One species of yeast is known, namely, saccharomyces apicu- 

 latus, which, while fermenting d-fructose and glucose, has no effect on galac- 

 tose. This yeast can therefore be used to isolate galactose from a mixture of 

 'the monosaccharides. It reduces Fehling's solution, its reducing power 

 being somewhat less than that of glucose. Yeasts can be trained to ferment 

 galactose. 



MANNOSE. Mannose, though an assimilable sugar, is of such rare occurrence in 

 our food-stuffs that it plays practically no part in animal physiology. It is dextro- 

 rotatory, reduces Fehling's solution, ferments easily with ordinary yeast, and gives 

 un osazone which is identical with that derived from glucose. 



DERIVATIVES OF THE HEXOSES 



Two derivatives of glucose are of considerable physiological importance; 

 namely, glucosamine and glycuronic acid. 



