8o THE CARBOHYDRATES 



salt. Filter once more, and evaporate the filtrate almost to 

 dryness, and acidify with nitric acid ; the precipitate is pure 

 mucic acid. 



MANNOSE. 



Mannose may be obtained by the hydrolysis of a form ot 

 mannane contained in salep mucilage (Orchis Morid] and from 

 several other so-called hemi-celluloses contained in peas, 

 coffee beans, date stones, etc. It is most conveniently prepared 

 by the hydrolysis of the hemi-cellulose contained in ivory 

 nuts, the fruits of Phytelephas macrocarpa ; turnings from these 

 seeds, obtained in the manufacture of vegetable ivory buttons, 

 are ground up and 150 gms. are added in portions to 1 50 gms. 

 of 75 per cent sulphuric acid, keeping the temperature below 

 40 C.* After standing for some hours the mixture is diluted 

 to two litres and gently boiled under a reflux condenser for 

 three hours. After carefully neutralizing with slaked lime, 

 the solution is filtered and decolorized by means of charcoal 

 and then by lead acetate followed by hydrogen sulphide ; it 

 is then evaporated in a vacuum to a syrupy consistency and 

 mixed with an equal volume of glacial acetic acid and allowed 

 to crystallize. 



Mannose has a sweet taste followed by a bitter one ; when 

 dry, it is a hard crumbling substance, which, however, de- 

 liquesces and is readily soluble in water ; it is only slightly 

 soluble in hot alcohol and is insoluble in ether. It is dextro- 

 rotatory, [a] D 20 = +14-36 in 10 per cent solution, and is 

 readily fermentable by yeast 



Detection. 



i. Mannose is most readily detected and estimated by 

 means of its phenylhydrazone, which is almost insoluble in 

 water, and forms almost at once on adding phenylhydrazine 

 acetate to an aqueous solution of the sugar ; the phenylhydra- 

 zone is soluble in a very large volume of boiling water, and 

 separates in fine prisms from the solution on cooling. These 

 crystals melt at 195-200. 



* Hudson and Sawyer: " J. Amer. Chem. Soc.," 1917, 39, 470. 



