132(5 
coHcenIralion, due to the vaiyiiig reaction velocities, occiir, which 
can onl}^ be equalised by the operation of diffiision or by mechanical 
mixing. On this account, it is evident that tlie velocity coefficients, 
which are obtained in any series of experiments can only represent 
average valnes, which are iiitlnenced by the particnlar conditions 
under whicli tlie 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 tliat this can be 
regarded as the expression of the general law of photo-kinetics. 
Lüther and Weigert (Zeit. Phys. Chem. 1905, 51, 297 ; 53, 385) 
are of opinion that the general equation is not applicable to rever- 
sible photo-chemical qhanges and these authors foinnulate the law in 
the following words : “Tlie quantity of a snbstance, sensitive to light, 
which nndergoes change in a given element of volume per unit of 
time, is proportional to the light absorbed during the same time by 
the snbstance 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 tbr raany catalysors in those changes. 
The reactions studied come in the category of Irreversible reactions 
i.e. group (2) of photochemical reactions (see page 1324). 
Ilhimination of the photochemical reactions studied. In all these 
observations direct sunlight or diffused day light was used. The light 
of the snn, whether in its direct form or as diffused day light, is the 
most available and immediate source and naturally is the normal to 
which, in regard to colour, artificial light source must approximate. 
Ex p eriinenta I pro cedm 'e . 
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 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.c. stoppered 
bottles were taken and cleansed ; they were rhen steamed for ^ an 
hour and dried. Exactly 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 (cf. Caldecott. Proc. Chem. Soc. (1904) p. 
199) in order to cut off all light rays. Genei-ally the bottle with 
