338 
1)R. MEYER WILDERMAN ON CHEMICAL DYNAMICS 
or molecular transformation also proves to be of a general and simple nature ; the 
velocity being directly proportional to the surface of contact of the reacting parts of 
the heterogeneous systems and to the remoteness of the system from the point of 
equilibrium.* The velocity of chemical reaction and chemical equilibrium in 
heterogeneous systems represent no phenomena sui generis, the laws concerning 
them being only combinations of the above two laws.f The laws relating to 
equilibrium found their rational explanation and foundation in the thermodynamic 
researches of Horstmann, and more fully in those of W. Gibbs and yan’t Hoff, 
whilst the laws applying to the velocity of reaction in homogeneous systems are the 
result of van’t Hoff’s thermodynamic considerations. 
In all the above researches the phenomena of the velocity of chemical reaction and 
of chemical equilibrium are the outcome of those intrinsic properties of matter, 
always existent in and inseparable from it, which we usually call chemical affinity or 
chemical potential. It is known, however, that a system can be brought into a state 
of reaction, and that new systems and new equilibria can be formed, when energy 
from an external source, such as light or electricity, is introduced into it. The effect 
of an electric current upon a chemical system, e.g., is determined by Faraday’s law 
of electrolysis, whilst the thermodynamic connexion between chemical and electrical 
(and gravitation) energy has been developed by W. Gibbs. 
The object of this investigation was to ascertain, if possible, the laws governing 
the velocity of chemical reaction and chemical equilibrium when this is caused by the 
introduction of light energy into the system. Is the velocity directly proportional to 
the amount of the light energy introduced or absorbed by the system in the unit of 
time, independent of the reacting masses or concentrations, i.e., is the law here 
analogous to that of Faraday for electrolysis, or is the velocity of reaction some 
function of the reacting masses ? What are the laws governing chemical equilibrium 
as affected by light ? It is evident that to furnish an answer to the above problems 
careful experiments bearing directly on the fundamental issues in question and 
a careful theoretical consideration of the results so obtained are absolutely needed. 
This is the more imperative as from the hundreds of reactions known to be caused or 
influenced by light| not half a dozen can be found suitable for quantitative measure¬ 
ments. 
It soon appeared that the chemical reaction chosen for the study of the laws of 
chemical kinetics must be very simple and as far as possible uncomplicated by 
secondary phenomena. The chemical action observed must be caused by light alone, 
and stop when light is removed. Bunsen and Boscoe’s reaction (H 2 + Cl 2 = 2HC1) 
seems to fulfil these conditions, but inasmuch as no change of volume takes place, no 
* See M. Wilderman, ‘Zeits. Physik. Chem.,’ 1899, 30, p. 341, and especially ‘Phil. Mag.,’1901, 
July, p. 50. 
1 See ‘ Zeits.,’ loc. cit., pp. 363-382. 
I See Edek’s ‘ Handbuch der Photographic.’ 
