STEREO-CHEMISTRV AND PHYSIOLOGY 471 



To save space in printing, this is usuall}- written in the 

 following way : 



H H 



Now, here, as in the case we have already dealt with, sub- 

 stitution may produce a difference in configuration ; for if we 

 take the case of the compound : 



CH, CH.R 



^CH . R 



we shall find that there are two possible spacial arrangements 

 of the atoms. In the first, we have the two groups R and one 

 hydrogen atom in the same plane, while in the second arrange- 

 ment we have one R group and two hydrogen atoms in the same 

 plane : 



H 



(I) ,C, ,C, (II) 



Form (I) is called the a's-f orm ; form (II) the frmts-f orm. 



In this section two instances are known in which the two 

 isomers differ from one another in smell. In each case the 

 cis-form has a stupefying odour, while the trans-isomer is 

 devoid of smell. The substances in question are the hexahydro- 

 o-diethyl-benzylamine carboxylic acids : 



CH.,-CH,. H CH,— CH., CHa.NCCHs)^ 



/ \ I / ' \ I 



CH3 H C CH., H C 



CH— C CH,,.N(CH3), CH3— C^ H 



I " I 



COOH COOH 



Cis-form Trans-form 



