1325 
be vegarded as the distingnishing characteristic of all photo-chemical 
reactiotis. From the data obtained in the experi mental investigation 
of a number of such reactions, it appears that tliese are in genei al 
iinimolecnlar and have a very small temperatnre coëfficiënt of 
velocity. Thns, Dkwar (Chem. News 84 , 281, 293 (1901) ) has shown 
that at the tempei’atnre of liquid air ( — 183°), photographic aclion 
is 20 7„ and at the temperatnre of liquid hydi'ogen ( — 250°), it is 
but 10 “/o of its valiie at ordinary temperatures ; but from VAN ’t Hofp’s 
rille we know that in ordinary Chemical reactions a rise of 10° 
doublés the velocity of the reaction. These facts have led to the 
view that the absorbed radiant energy is not directly responsible 
for the Chemical cliange 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 molecnlar complexes 
which act as reaction nuclei (cf. Dhar, Zeit. Elektrochem. 1914, 20, 
57) and (Weigert, 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 obvioiis that the rate of formation of the 
products of the photo-chemical change will be delermined by the 
speed at which the preliminary light change occiirs; as lias already 
been remarked in the work on snpersaturation that the catalyst 
only acts as a nucleus to a change which pioceeds by its inherent 
forces, this also applies to the catalytic intluence of light. 
The law of mass action has been first applied in a especial form 
by WiTTiWER (Fogg. 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 kinetic eqnation 
V = — = k ... — K' cP d<l 
dt 
in which a, h, c, ... d .. . etc. 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 K and K' are the 
velocity coefficients of the two opposed reactions. The values of K 
and K' 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 absorptioii, the light intensity \aries from point 
to point of the reaction mixture with the result that differences in 
