309 
and these reactions when investigated would show great sensitiveness 
to light (Zeit. Phys. Chem. 1909, 67, 129). 
Cain and Nicorr (Jour. Chem. Soc. 1903, 83, 470) have proved 
that the decomposition of the diazo salts has the value of about 5 
for their temperature coefficients and it is well known that diazo 
salts are sensitive to light. 
PENDLEBURY and Srwarp (Proc. Roy. Soc. 1889, 95, 396) have 
shown that the reaction between KCIO, KI and HCI has the temper- 
ature coefficient of about 4 in the dark and I have found that the 
reaction is very sensitive to light. 
I have already shown (Trans. Chem. Soc. 1917, 111, 707) that 
the oxidations of sodium formate by mercuric chloride and by iodine 
has 4.05 for their temperature coefficient. These two reactions are 
also very sensitive to light (Duar, Proc. Akad. Wet. Amsterdam 
1916, 24, 1324). 
I have shown that light markedly accelerates the reaction between 
iodine and oxalates and now I have tried to find out which part 
of the spectrum is active in this reaction. For this I exposed five 
small tubes containing a mixture of potassium oxalate and iodine 
in a spectrum obtained from a carbon are-light. It was found that 
the iodine disappeared first in the tube held in the indigo portion 
near the violet end of the spectrum. Then that of the tube held in 
the blue region. The chemical change took place almost simultane- 
ously in the tubes held in the green and violet portions of the 
spectrum and the colour of the tube in the red end was the last 
to disappear. 
A mixture of mercuric chloride and potassium oxalate undergoes 
the following change in sunlight: 
2HeCl, + K,C,0, = 2KCl + 2CO, + 2HgCl. 
This decomposition can also be induced by lights obtained from a 
carbon arc, quartz mercury vapour lamp, and an arc obtained by 
passing alternating current in electrodes made of thorium and 
zirconium oxides. I have repeatedly observed that in tropical sunlight, 
a solution of ammonium cupric oxalate decomposes readily in glass 
vessels with the separation of metallic copper and evolution of 
carbon dioxide; but it was found impossible to bring forth this 
change by light obtained from the carbon arc or the thorium and 
zirconium oxides arc. 
] have found that uranium nitrate markedly accelerates the photo- 
chemical decomposition of a mixture of mercuric chloride and potas- 
sium oxalate, but chromates having the same yellow colour as the 
