REVERSIBLE AND IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 389 
systems follow (in the dark), as a mathematical necessity, from two well-established 
laws—-one governing the speed of reaction in homogeneous parts of the system (the 
law of mass action) and the other the speed of molecular reactions (speed of solution 
or evaporation and of separation or condensation of solids or liquids in the system) 
between the different parts in the system (a law established by myself). After the 
work of Helmholtz and Planck, and others who find themselves constrained to 
assume that every substance has a solution pressure or vapour pressure, in order to 
explain the phenomena of formation of electrical potential in a galvanic combination, 
&c., the above treatment of the subject becomes an unavoidable necessity. 
Messrs. Nernst and Brunner therefore should not have applied a law which I 
established for molecular reactions between the same substance in different parts of 
the heterogeneous system to the velocity of chemical reaction in heterogeneous 
systems, which is a conglomerate of reactions. The reactions investigated by Nernst 
and Brunner are only particular instances of my laws and equations for chemical 
reactions in heterogeneous systems, and any conceivable values may be taken for 
each member of the equations without impairing in the least degree their validity. 
If the numerous assumptions of Nernst and Brunner be true, they must therefore 
follow as a particular case of my equations; but, as I will show at another place, 
both their assumptions and experiments are arbitrary. 
In conclusion I wish to express my thanks to the Managers of the Boyal Institution 
for enabling me to make use of the resources of the Davy-Faraday Laboratory, and 
especially to Dr. Ludwig Mond, who by his kind assistance and interest in my 
work enabled me to undertake and to carry out the above research. 
