120 MOLECULAR STRUCTURE 



There arise therefore four types, each including a pair of 

 antipodes : 



i, 8 2, 7 3, 6 4, 5. 



As an example we may take the pentoses 

 CH 2 OH(CHOH) 3 COH, 

 and considering only the types, we have 



CH 9 OH 



Actually these four types are found, in arabinose, ribose, 

 xylose, and lyxose, of which arabinose is known in the two 

 oppositely active forms, while of the other three types one 

 antipode of each is wanting. 



A definite choice of one of the four formulae, say for 

 arabinose, may now be made on the following two 

 grounds : 



1. Arabinose is oxydized to a glutaric acid, 



COOH(CHOH) 3 COOH, 



which is active. In this way the possibilities Nos. (i, 8), 

 and Nos. (3, 6) are excluded, since they would lead to 

 a glutaric acid with symmetrical configuration, and there- 

 fore of the inactive undecomposable type. 



2. Glucose and mannose are formed from arabinose by 

 replacing the group 



COH 



with HCOH 

 COH, 



two isomers of the formula CH 2 OH(CHOH) 4 COH being 

 produced ; first, according to Kiliani and Fischer, hydro- 

 cyanic acid is added, the iiitril produced converted into 



