490 
uranium sall exert a markedly negative effect. Mathews and Weeks 
(Journ. Amer. Chem. Soc. 1917, 39 , 635) have shown tliat uranium 
nitrate is also a positive calalysl in the photocheraical oxidation of 
sodinm sulphite. Moi’eo\ er, it is wellknovvn that uranium salts mar- 
kedly help the photochemical decomposition of organic aeids (e.g.), 
oxalic, formie, lacffic etc. Hence it appears that an uranium salt is 
a positive cataljst of great generality in pliotocliemical reactions. 
I have observed that manganese sulphate exerts a negative effect 
in tlie pliotocliemical decomposition of a mixture of mercuric chloride 
and potassinm oxalate. It has already been shown that manganese 
salts act as a negative catalyst in the reactions between phosphorous 
and chroniic acids, formie and chromic acids, mercuric chloride and 
sodium formate, iodine and sodiuni formate, silver nitrate and sodiuni 
formate, silver nitrate and ferrousammonium sulphate, etc. 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 lighis thus obtained and the results obtained 
are summarised below : 
(1) HgCl, + (NH,),C,(), 
f2) I, + (NBJ,CA ^ 
(3) FeCl, + (NHJ, C,0„ 
i4) Pyrogallol and Pyrogallate 
-(- O, 
(5) Hydroquinone O, — 1 >- 
(6) CujCl, (ammoniacal or acid) 
4-03-» 
(7) Decomposition of H^S -» 
(8) Quinine acid sulphate -|- 
H,Cr 30 , ^ 
(9) KMnO, H3C,0, ^ 
Blue, violet and ultra-violet more 
active than the red and infra-red. 
[ practically uniform acceleration in 
I different jiarts 
) Blue and violet slightly more active 
) than the red. 
i Violet and ultra-violet more active 
than red. 
In a remarkable article Pekhin (Annales de Physique 1919, t. XI, 1) 
has enunciated the following hypothesis: 
“A11 Chemical reactions are provoked by light radiations. Their 
velocities are determined by the intensity of the light radiations and 
depend on teniperature to such an extent as the light intensity 
depends on temperature”. By applying the idea of the emissive 
power of perfectly black bodies and ils relation to temperature, 
