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uranium salt exert a markedly negative effect. Matuews and Weeks 
(Journ. Amer. Chem. Soc. 1917, 39, 635) have shown that uranium 
nitrate is also a positive catalyst in the photochemical oxidation of 
sodinm sulphite. Moreover, it is wellknown that uranium salts mark- 
edly help the photochemical decomposition of organic acids (e.g.), 
oxalic, formic, lactic ete. Hence it appears that a uranium salt is 
a positive catalyst of great generality in photochemical reactions. 
I have observed that manganese sulphate exerts a negative effect 
in the photochemical decomposition of a mixture of mercuric chloride 
and potassium oxalate. It has already been shown that manganese 
salts act as a negative catalyst in the reactions between phosphorous 
and chromic acids, formic and chromic acids, mercuric chloride and 
sodium formate, iodine and sodium formate, silver nitrate and sodium 
formate, silver nitrate and ferrousammonium sulphate, ete. So it 
seems that a manganese salt is a negative catalyst for light and 
dark reactions alike. 
I have also observed the effects of the different parts of the 
spectrum on several other photochemical reactions by passing ordinary 
sunlight through different solutions and exposing the reacting sub- 
stances to the filtered lights thus obtained and the results obtained 
are summarised below: 
(1) HgCl, +(NH,),C,0, — 
(2) I, + (NH), CO, — 
(3) FeCl, + (NH),C,0, — 
(4) Pyrogallol and Pyrogallate 
+ 0, => 
(5) Hydroquinone + O, — | practically uniform acceleration in 
(6) Cu,Cl, (ammoniacaloracid) ( different parts 
Hd 0, => 
(7) Decomposition of H,S — 
(8) Quinine acid sulphate + Blue and violet slightly more active 
H,Cr,O, — J) than the red. 
Violet and ultra-violet more active 
Oene than red. 
Blue, violet and ultra-violet more 
active than the red and infra-red. 
In a remarkable article Perrin (Annales de Physique 1919, t. XI, 1) 
has enunciated the following hypothesis: 
“All chemical reactions are provoked by light radiations. Their 
velocities are determined by the intensity of the light radiations and 
depend on temperature to such an extent as the light intensity 
depends on temperature”. By applying the idea of the emissive 
power of perfectly black bodies and its relation to temperature, 
