394 
DR. MEYER WILDERMAN ON CHEMICAL DYNAMICS 
know of any reversible system, in which the two opposite reactions do not go on in 
the dark but go on in the light. But we do know systems in which one reaction 
goes on in the dark and is modified in light, while the opposite reaction goes on in 
the light only, e.g ., 
2AgCl in solution !_; Ag 0 (or Ag,Cl) in sol. -f- Cl a in sol. 
it 'it 
AgCl solid Ag (or AgoCl) solid. 
Here is an homogeneous system, on the one hand silver chloride is decomposed by 
light into silver and chlorine or silver sub-chloride and chlorine (this question is at 
present unsettled), on the other hand silver (or silver sub-chloride) and chlorine 
combine in the dark forming silver chloride, and this combination evidently goes on 
in light also (though probably with a different speed). Let the volume of the solution 
be Y. For the first reaction in light we have according to the law found above 
dx (A - 
~T — c -V 
CIT V 
_ /JQ 
— , wdrere - is the concentration of the molecules of silver chloride in 
solution (however small this may be) at the time r, and x/v is the concentration of 
the chlorine as well as of the silver molecules formed in solution; c is the velocity 
constant which changes with the intensity and composition of the light passing 
through the system. For the second reaction we have according to the law of mass 
, where c is the velocity constant for the reaction in 
which silver chloride is formed from silver and chlorine. The velocity constant in 
light is different from that in the dark, say c", however small this difference may be, 
for the reason that chlorine and silver are in a different state of energy in the dark 
and in the light. It follows from this that when equilibrium takes place in the light, 
or when no further variation in the masses takes place, 
action in the dark (— ) = c' - 
dx 
A / 
dr 
x 
V 
dx _ (ch:d 
tI t \dr) 
= 0 
c (A - A 2 
(A - xy 
c 
that is, we must at the point of equilibrium get a constant K, which will regulate the 
masses forming the reversible system with the variation of the volume or of the 
concentrations or of the partial pressures of the substances, because both opposite 
reactions have each a separate velocity constant before equilibrium. Though this 
proof of the necessity of the existence of a constant of equilibrium is absolute, it 
would have been very valuable and desirable to directly illustrate the equilibrium 
constant K in a reversible system from the varying masses at equilibrium, as we 
succeeded in doing for the velocity of reaction. Unfortunately there is not one 
homogeneous system known where such a proof could be successfully carried out. It 
is well to remember the enormous difficulties one meets with in this region, when even 
such apparently simple reactions as the combination of carbon monoxide and chlorine 
