THE SPACE RELATIONS OF ATOMS. 145 



noted which may also be said to depend on the principle of 



R'CR" 



least disturbance. The molecule T3 ,_ r( „ will suffer less dis- 



RCR' 



CR" R'f^T?" 



turbance in becoming than will the molecule , ; 



& CR R'CR' 



and accordingly it has been found by Wislicenus and his 



pupils that the rate of reduction in the same conditions is 



greater in the former case. 



With regard to the mutual action of the R groups, we 



may say that the formula showing those groups which have 



the greater attraction for each other as occupying the same 



side of the molecule, will belong to the more stable isomer. 



Wislicenus assumed that the degree of attraction varied 



directly with the difference in positivity of the radicals R. 



But the matter appears to be not so simple. If, however, 



we have reason to assign to the less stable isomer the for- 



R iv CR'"' m ° re stable R'"CR iv ' We may tGSt 



this by determining the heat of formation, which will always 

 be greater for that isomer which is in an absolute sense the 

 more stable ; the proportion of this isomer formed at any 

 given temperature may not be the greater, but as the tem- 

 perature is lowered its proportion will increase until ulti- 

 mately it preponderates. 



Again when two groups in a compound readily react 

 together, we attribute to them the "cis" position; hence, 

 from its tendency to form the anhydride, maleic acid must 

 have this formula. The same conclusion may be drawn 

 from its formation from ring-compounds ; and that the 

 negative groups are closer together in this compound than 

 in its isomer is shown by the fact that its dissociation constant 

 is twelve times higher. 



The application of the tetrahedron-hypothesis to ring- 

 compounds is illustrated by the figures of trimethylene 

 derivatives, published by Van't Hoff twenty years ago ; 

 here the R groups occupy two planes, one on each side 

 of that of the carbons ; thus the arrangement for the 

 trimethylene dicarboxylic acids which Buchner deduced, as 

 we have seen, independently of the tetrahedron-hypothesis, 



