1099 
concentration, due to the varying reaction velocities, oecur, which 
can only be equalised by the operation of diffusion or by mechanical 
mixing. On this account, it is evident that the velocity coefficients, 
which are obtained in any series of experiments can only represent 
average values, which are influenced by the particular conditions 
under which the reaction is allowed to take place. Although in the 
case of certain non-reversible changes the observations of the rate 
of change appear to be in satisfactory agreement with the above 
general equation of Nernst, it seems improbable that this can be 
regarded as the expression of the general law of photo-kinetics. 
Lurner and Welcrrt (Zeit. Phys. Chem. 1905, 51, 297 ; 58, 385) 
are of opinion that the general equation is not applicable to rever- 
sible photo-chemical changes and these authors formulate the law in 
the following words: “The quantity of a substance, sensitive to light, 
which undergoes change in a given element of volume per unit of 
time, is proportional to the light absorbed during the same time by 
the substance contained in this volume element.” 
This is the general aspect of photochemical changes. 
In the present investigation it is shown that light acts as a positive 
catalyst to a series of chemical reactions and that light may be 
substituted for many catalysors in those changes. 
The reactions studied come in the category of irreversible reactions 
i.e. group (2) of photochemical reactions (see page 1097). 
Ilhimination of the photochemical reactions studied. In all these 
observations direct sunlight or diffused daylight was used. The light 
of the sun, whether in its direct form or as diffused daylight, is the 
most available and immediate source and naturally is the normal to 
which, in regard to colour, artificial light source must approximate. 
Experimental procedure. 
It is self-evident that the chemical intensity of sunlight would 
depend on the sun’s altitude. So in the present investigation, the 
difficulties, due to change of the sun’s altitude, were avoided by a con- 
comitant blank experiment carried out under exactly similar condi- 
tions in darkness. Thus two similar thin glass 100 c.e. stoppered 
bottles were taken and cleansed ; they were then steamed for half an 
hour and dried. Exactly the same volume of solution was added to each 
of them and one was exposed to light, while the other was either 
covered with a piece of thick black flannel or pasted with a thick 
coating of black Japan (ef. Carprcorr. Proc. Chem. Soc. (1904) p. 
199) in order to cut off all light rays. Generally the bottle with 
