THE CHEMISTRY OF THE SUGARS 91 



stated generally that the researches have been conducted on 

 the following lines : — 



{a) The production of aldoses or ketoses from naturally 

 occurring substances whose ordinary constitutional formulae 

 and specific optical activity are known. Thus dextro mannite 

 or mannitol, which can be readily obtained from naturally 

 occurring manna, yields dextro mannose on oxidation. 



(&) The buUding up of sugars from compounds of known 

 constitution by Kihani's reactions. Thus arabinose yields 

 eventually Z-glucose or ?-mannose as follows : — 



(c) Passing from ketose to aldose or vice versa by means of 

 the phenyl- hydrazine compounds ; the example already given 

 on p. 87 •will illustrate this. 



(d) Eesolving inactive compounds by fractional crystallisa- 

 tion of suitable salts, or by the action of enzymes. 



It will be useful briefly to describe the chief properties of 

 one or two well-known members of the hexose group and of 

 certain related compounds, which are of interest from the fact 

 that they have been used as a means of differentiating certain 

 bacteria one from another, by the capacity or otherwise which 

 these may possess of fermenting the substance in question. 



Mannite or Mannitol has already been referred to ; it is a 6 

 carbon alcohol of the general formula CH30H{CHOH)^CH20H. 

 It occurs in manna, which consists of the evaporated sap 

 exuding from various species of ash cultivated in southern 

 Europe ; it also occurs widely distributed in the vegetable 



