no AIR. P. V. BEVAN OX THE COMBIX.ATTOX OF HYl)ROC4P:X 
consider the question of unsaturated and molecular compounds, and first of all, the 
doctrine of Valency. This doctrine, in spite of its very great use in the systemati¬ 
zation of organic chemistry, has to some extent retarded progress by affording too 
rigid a view of chemical compounds. Until we possess a dynamical concej)tion of a 
molecide, the theory of bonds can only be regarded as a method of exj^ression of 
general facts, and not in the sense of giving a representation of actually existing 
links between atoms. Valency at present has no bearing on compounds which have 
been called molecular. But no line can be drawn between these comj)Ounds and 
definite atomic compounds. The fact of the existence of such controversies as that 
as to whether phosphorus penta-chloride was an atomic compound or a molecular 
compound of phosphorus trichloride and chlorine shows that there is really no 
difference in kind between these two classes of compounds. Compounds which are 
saturated under certain circumstances can under other circumstances combine with 
other systems, so that the difference between saturated and unsaturated compounds 
is merely one of degree. “ Apparently the only saturated molecules known to us at 
present, molecules destitute of all power of exercising an external influence, are 
those of gases such as helium and argon.” There is therefore in all molecules, 
excepting those of the inert gases of the atmosphere, a tendency to form more 
complicated molecules, some residual affinity which is in many cases unaccounted 
for on the Valency theory. The difference tlien between molecular and atomic 
compounds is a difference of stal)ility under certain conditions, and we must recognise 
that it is possible for additive compounds of all degrees of complexity to exist, 
provided that the conditions are suitable. In this connection, the views of NefI 
with regard to certain organic substitutions are of importance, as suggesting an 
explanation of reactions which can l)e applied to gaseous systems with which we are 
specially dealing. Nef supposes that reactions take place l)y unsaturated molecules— 
compounds exhibiting double linking, or compounds with unsaturated atoms—forming- 
additive compounds with other molecules ; afterwards this additive compound breaks 
down into the final product of the action. For agreement with the Valency theory, 
Nef supposes that one of the compounds, at the moment of formation of the additive 
molecule, is dissociated. For example, consider the action of bromine on benzene. 
In this case, bromine is the unsaturated molecule 
Br = Br. 
This combines with the benzene molecule, which undero-oes a dissociation at the 
moment of combination, forming the molecule 
(J.H^ - Br = Br - H. 
Armstrono, ‘ Encyc. Brit.,’Hupjjlement, 1902, Art. “Chemistry.’ 
t Nef, ‘ Liebig’s Annalen,’ 1897, vol. 202. 
