AND CHLORINE UNDER THE INFLUENCE OF LIGHT. 
Ill 
And finally this molecule breaks clown into the more stable condition 
CgHj — Br and HBr. 
There is no necessity for the action to be brought into formal agreement with the 
theory of Valency. We can think of the benzene molecule forming a molecular 
compound with the bromine molecule, in which system the individual atoms can 
exert forces on each other, so that an internal re-arrangement of atoms in the 
additive molecule can take place, resulting in an unstable molecule which breaks 
down into the final products. This conception gives us a reasonable picture of the 
process of substitution. We do not consider the atoms in a molecule as rigidly 
attached at certain definite positions with regard to the molecule, but whatever the 
forces between atoms may be, in the molecule all atoms can act on each other. Thus 
if a bromine molecule as a whole be brought into close enough proximity to the 
benzene molecule, then a bromine atom and a hydrogen atom form, at first as part 
of the molecule, a system without much effect external to itself and so without much 
connection with the molecule, in other words, the complex additive molecule is 
unstable and the HBr molecule splits off 
This view is in effect very much that of Kekule* on double decomposition. If 
two bodies aa\ hh' react, giving ah, a'h', then at first there is a union and afterwards 
a decomposition. Kekule represents the process thus :— 
a b ah 
a' y a'h' 
before action, during action, after action. 
A somewhat similar view is that j^roposed by Armstrong, but here a group of 
three species, forming a voltaic circuit, is introduced, which seems unnecessary and 
gives really no simplification. Armstrong extends his view to include all actions. 
But actions which occur in solution are very much more simply explained, on the 
theory of electrolytic dissociation, as direct combinations. There are probably in 
gaseous systems cases of simple combination, though in the great majority of cases 
the action is more complex. An example of this simple combination in gaseous 
systems is probably to be found in the case of the action of chlorine on mercury. The 
action occurs when the substances are perfectly pure and dry (Shenstone), and there 
appears to be no reason to suppose that anything except direct combination takes 
place. But in very many gaseous actions the direct combination does not take place, 
at any rate with measurable velocity, and some catalyser is necessary. The experi¬ 
ments of Baker, Dixon and others on the behaviour of pure gases show that in most 
* Kekule, ‘Liebig’s Annalen,’ 1858. 
