1902.] 



and Statics under the Action of Light. 



67 



reaction in heterogeneous systems and chemical equilibrium in hetero- 

 geneous systems represent no phenomena sui generis, and the laws 

 governing them are only combinations of the other two laws 

 mentioned.* The laws concerning equilibrium have found their 

 rational explanation and foundation in the researches of Horstmann, 

 and foremost of W. Gibbs and Van't Hoff; the laws concerning 

 velocity of reaction in homogeneous systems in the thermodynamic 

 considerations of Van't Hoff. In all the above researches the phe- 

 nomena of chemical reaction and of chemical equilibrium are the out- 

 come of the intrinsic properties of matter, always existent in and 

 inseparable from the same, and which we usually call chemical affinity 

 or chemical potential. 



But, as we know, a system can also be brought into a state of 

 reaction, new systems, new equilibria, can be formed when energy 

 from an external source, such as light or electricity, is introduced into 

 the same. The effect of an electric current upon a chemical system is 

 given by Faraday's law for electrolysis. The thermodynamic con- 

 nection between chemical and electrical (and gravitation) energy is 

 given by W. Gibbs. 



What are now the laws governing the velocity of chemical reaction 

 and chemical equilibrium when they are caused (not only influenced) 

 by the introduction of light 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., have we to expect here a law analogous to that 

 of Faraday for electrolysis 1 or is the velocity of reaction some func- 

 tion of the reacting masses 1 What are the laws governing chemical 

 equilibrium in light 1 It is evident that to furnish an answer to these 

 problems careful experiments, quantitative measurements bearing 

 straight upon the fundamental issues in question, were absolutely 

 needed, and the help of the principles of thermodynamics in giving a 

 rational explanation to the fundamental facts elicited by experiments 

 had to be sought for. 



The experimental work, started at the Davy-Faraday laboratory of 

 the Eoyal Institution, in 1898, proved to be of very great difficulty, 

 resolving itself into the solution of a series of other problems lying 

 beyond the region of its immediate purpose, and only after two and a- 

 half years of preliminary work the author succeeded for the first time 

 in 'getting results, which allowed sure insight into the nature of the 

 phenomena under consideration. 



From the hundreds of reactions which are known to be caused or 

 influenced by light,! there are not half a dozen which are suitable for 

 quantitative measurements. The reaction used must be caused and 



* See the same paper, ' Zeits. fur Phys. Cliemie,' 1899. 

 f Eder's ' Handbuch der Photographic' 



