AND STATICS UNDER THE INFLUENCE OF LIGHT. 
893 
hydrochloric acid, but chlorine and water first form an intermediate compound 
hypochlorous acid (and hydrogen), which with hydrogen forms the hydrochloric acid, 
setting free the same molecules of water which are again used in the reaction, i.e., 
instead of having H 2 -f- C1. 3 = 2HCL, we have Cl 2 + H 2 0 = CLO -f H 3 ; CLO + H 4 
= 2HC1 + HoO. I doubt the correctness of this explanation for the following 
reason :—It is known that chlorine and water, when exposed to light, form not 
hypochlorous acid but hydrochloric acid (Wittwer, Bunsen, and Roscoe).^ Still 
the difficulty remains that chlorine and hydrogen are not the only system having 
such properties. The system chlorine and carbon monoxide exhibits the same 
property. We also know that ammonia and hydrochloric acid do not combine 
when perfectly dry; carbon monoxide and hydrogen do not explode when perfectly 
dry (Dixon) ; perfectly dry hydrogen peroxide does not act upon a photographic 
plate (Russell). For this reason it seems that in gaseous systems a phenomenon is 
met which in other cases is called “catalytic action,'’ in which a reaction is accelerated 
or caused by the presence of an extraneous substance, which apparently or in reality 
takes no part in the reaction, e.g., the action of platinum-black upon the decompo¬ 
sition of hydrogen peroxide, &c. 
The catalytic action of gases upon the velocity of reaction in the gaseous systems 
may be divided into “catalytic action with an accelerating influence” (to this belongs 
the action of water vapour upon the gaseous systems mentioned above) and into 
“ catalytic action with a retarding influence” (to this belongs the action of oxygen or 
air upon the systems chlorine and hydrogen or chlorine and carbon monoxide). This 
division is, however, of a purely formal nature, and hardly anything is known of the 
ultimate nature of the phenomenon. 
Velocity of Chemical Reaction and Chemical Equilibrium in Light. 
Having thus established beyond any doubt that the velocity of reaction in light is 
governed in homogeneous systems by the same law of mass action as in the dark, the 
influence of temperature as well as of the intensity of light upon the value of the 
velocity constant, as well as the connexion between the velocity constant and the 
wave length on the monochromatic light, have still to be investigated; and finally 
the investigation of heterogeneous systems regulated by other fundamental principles 
remains necessary. This will form the subject of the author’s future investigation. 
Ou the other hand the solution of the problem for chemical kinetics in light 
evidently already shows with perfect certainty that the law of mass action, which 
governs chemical equilibrium in homogeneous systems in the dark must necessarily 
also govern chemical equilibrium in the light. It is most remarkable that we do not 
* Professor Dixon, in a private communication, makes a better suggestion—that the reaction between 
hydrogen and chlorine is to be conceived thus :—CL + ILO = 2HC1 + 0 (nascent); O l- LL = H l .O, 
the first equation being reversible. 
VOL. CXCIX.-A. 3 E 
