Chemical Constituents of the Animal Body. 201 



polarized light in one direction exactly to the same 

 amount that the other member rotates it in the opposite 

 direction. It is not possible in this place to give in detail 

 the methods by means of which the relative positions of 

 the hydroxyl and hydrogen groups in the above isomerides 

 have been ascertained. The present knowledge on this 

 subject is due chiefly to the labours of Emil Fischer and 

 his pupils, whose work is published in a monograph of 

 extensive character. It will suffice to give an account of 

 a few of the reactions which have led to the final results, 

 and which indicate the stereometric relationships of the 

 sugars to one another. 



I. It is possible to reverse the positions of the hydrogen 

 and hydroxyl groups attached to the a-carbon atom (the 

 positions of the carbon atoms are reckoned from the aldehyde 

 group) that is, convert, e.g., a sugar 



CHO 



CHO 



H 



H- 

 OH 



OH- 



OH 

 OH 

 H 



into 



H 



OH- 



OH- 

 OH- 



CH 2 OH 



H 

 -OH 

 H 

 -H 



that is, convert a mannose into a glucose. 



These sugars are converted on oxidation by mild oxidiz- 

 ing reagents into acids, the CHO group being oxi- 

 dized to the COOH group. From glucose the acid 

 COOH(CH(OH)) 4 CH 2 OH can be obtained. On heating 

 such an acid with water to 150, made alkaline with 

 pyridine or quinoline, the position of the H and OH 

 on the a-carbon atom is reversed ; that is-^ 



