1098 
be regarded as the distinguishing characteristic of all photo-chemical 
reactions. From the data obtained in the experimental investigation 
of a number of such reactions, it appears that these are in general 
unimolecular and have a very small temperature coefficient of 
velocity. Thus, Dewar (Chem. News 84, 281, 293 (1901) ) has shown 
that at the temperature of liquid air (— 188°), photographie action 
is 20°/, and at the temperature of liquid hydrogen (— 250°), it is 
but 10°), of its value at ordinary temperatures; but from van ’r Horr’s 
rule we know that in ordinary chemical reactions a rise of 10° 
doubles the. velocity of the reaction. These facts have led to the 
view that the absorbed radiant energy is not directly responsible 
for the chemical change, but that its action consists in a preliminary 
transformation of the reacting system. This change, which may consist 
in the intramolecular transformation of the molecules of the light 
absorbing substance or in the formation of molecular complexes 
which act as reaction nuclei (ef. Duar, Zeit. Elektrochem. 1914, 20, 
57) and (Wricert, Ann. Physik. 1907 (IV) 24, 243) is then followed 
by the chemical reaction proper and if the speed of the latter is 
relatively very large it is obvious that the rate of formation of the 
products of the photo-chemical change will be determined by the 
speed at which the preliminary light change occurs; as has already 
been remarked in the work on supersaturation that the catalyst 
only acts as a nucleus to a change which proceeds by its inherent 
forces, this also applies to the catalytic influence of light. 
The law of mass action has been first applied in a special form 
by Wirriwer (Pogg. Ann. 97, 304 (1856) in his work on the 
influence of light on chlorine water and has been generalised by 
Nernst (Theoretical Chemistry 4 edition p. 732) in homogeneous 
systems. In a homogeneous system the velocity of the reaction at 
any moment will be given by the kinetie equation 
= ae = kambr,..— Ker aq 
dt 
in which a, 6, ¢,...d...ete. are the concentrations of the reacting 
substances m, n...p, q..- the number of molecules of the several 
substances actually involved in the change and A and A’ are the 
velocity coefficients of the two opposed reactions. The values of A 
and A’ depend on the intensity of the light acting on the system, 
and for light of the same kind, are, in certain cases at any rate 
proportional to the intensity. 
In consequence of absorption, the light intensity varies from point 
to point of the reaction mixture with the result that differences in 
