250 SCIENCE PROGRESS 



of the atom, measured by the maximum number of certain 

 atoms which the grouping atom can hold in stable union, 

 arose the conception of ordered arrangement within the mole- 

 cule. The discovery of isomerism associated such ordered 

 structure with definite properties. 



The idea of structure led to a system of schematic repre- 

 sentation in which the molecules were depicted as frameworks 

 in which valency was represented by lines or strokes proceed- 

 ing from the atoms as though they were mechanical means of 

 attachment. 



Formulae thus constituted were especially serviceable in 

 organic chemistry, but they introduced a belief or tacit assump- 

 tion that the attraction bringing about combination was 

 divided up into a definite number of equal parts acting in 

 definite directions from particular points upon the atom. 

 Deviation from these fixed directions was only possible with 

 loss of attractive force, strain and consequent looser attach- 

 ment. Further, since such forces could not exist without 

 some corresponding force to neutralise them, this, naturally, 

 led to the conception of latent valencies as an explanation of 

 the phenomena of variable valency. The forces unexerted 

 upon other atoms were regarded as mutually satisfying each 

 other. 



In recent years, following Werner, the tendency among 

 chemists has been to abandon these ideas and to revert to the 

 older and simpler conception of affinity as a force exerted 

 uniformly outwards from atoms and balanced by the similarly 

 exerted attraction of other atoms in combination with them. 



Any association of atoms may therefore possess affinity — 

 that is, be capable of exerting attraction upon other atoms or 

 groups. Consequently, there is no reason why a molecule 

 should not replace an atom, or vice versa. 



It follows naturally that there must be a space-limit to the 

 number of atoms or molecules which any given atom can 

 associate or hold together, and this must be connected with 

 the actual volumes of the atoms or with the spaces over which 

 they exert an effective influence, thus preventing other atoms 

 or groups from entering. 



Further, as the attractive force must diminish in some 

 way as the distance separating the centres of the attracting 

 atoms increases, the union can only be stable if this distance 



