90 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



are capable of reducing certain metallic salts with production 

 of the metal or a lower oxide of the metal. Thus, an ammoniacal 

 solution of nitrate of silver when warmed with ordinary glucose 

 (grape sugar) yields a briUiant mirror of silver. 



On warming with an alkaUne solution of copper potassium 

 tartrate (known as Fehling solution) a red precipitate of 

 cuprous oxide, CugO, is produced. This is an important 

 reaction which can be used for the quantitative determina- 

 tion of the amount of reducing sugar present in a solution. 



Stereo-isomerism of the Ketoses and Aldoses.— If the 



formula for an aldose or ketose containing more than two 

 carbon atoms be carefuUy studied, it will be seen that in most 

 cases one or more asymmetric carbon atoms are present in the 

 molecule. Thus to take the simplest case, viz., the aldose 

 form of glycerose CHgOH C HOHCHO, the centre carbon 

 atom is combined with four different atoms or groups, and 

 consequently a right-hand and left-hand and also an inactive 

 form of this sugar are capable of existence. 



In the case of a hexose the number of asymmetric carbon 

 atoms, and consequently of right-hand and left-hand forms, 

 becomes considerable ; thus a ketohexose contains three asym- 

 metric carbon atoms : — 



CH2OH C HOH C HOH C HOHCOCHgOH 



An aldohexose contains four asymmetric carbon atoms :— 

 CH2OH C HOH C HOH C HOH C HOH 



The separation and identification of the large number of 

 possible ketohexoses and aldohexoses is a very comphcated 

 task ; mainly by the exertions of Emil Fischer and his pupils 

 it has to a great extent been accomphshed. 



It would lead too far, and would be foreign to the subject 

 of the present work, to consider the methods of preparation and 

 identification of all these compounds in detail. It may be 



