1106 
the change, then the photo-chemical effect will be measured by a 
definite quantity of B produced from A. This effect, which may be 
termed E, should always correspond to the same amount of light 
energy. 
The total amount of light or “Light flux” over a time ¢ will be 
equal to /¢, where / is the intensity. Then the photo-chemical effect 
FE should be proportional to this, ie. A= A/t. This is the “Reci- 
procity law”, which states that the same photo-chemical effect is 
obtained with a light sensitive reaction for the same amount of 
light, whether the intensity be diminished and the time proportion- 
ately increased or conversely. 
It will also be seen that in sunlight the difference between the 
bromine that remains unchanged in the exposed and covered bottles 
is very marked, whilst in diffused light the difference is not so 
very marked. 
The oxidation of ethylaleohol by bromine in dilute aqueous solution 
has been investigated by BuGarszky (Zeit. Phys. Chem. (1901) 38, 
561; (1904), 48, 63; (1910). 71, 705) and he has found that the 
reaction takes place in consecutive stages, as represented by the 
formulae . 
C,H,OOH + Br, = CH,COH+2HBr.. . . . (D 
CH,COH + Br, + H,O = CH,COOH + 2HBr. . . (II) 
But he has not examined the effect of light on this change. It 
has now been observed that the change takes place more rapidly 
in sunlight and diffused daylight from the following tables. Similarly 
Bromine and ethyl-alcohol. 
| | Amount of | ‘ | 
5 i b i U d 5 
Quality of light. | mena | Ed | kerke | Volume of solution. 
| | in grams. 
a) Sunlight 158’ | 0.06527 0.01545 
5 c.c. ethyl-alcohol and 
Darkness 158’ Do 0.06412 10 c.c bromine water. 
10 c.c. ethyl-alcohol and 
Darkness 286’ Do 0.04628 10 c.c. bromine water. 
c) Diffused light 159’ Do 0.02895 | 5 cc. ethyl-alcohol and 
Darkness 159’ Do 0.06248 10 c.c. bromine water. 
d) Diffused light 285’ Do 0.02012 
Darkness 285’ Do | 0.04613 
10 c.c. ethyl-alcohal and 
b) Sunlight 286/ Do 0.01084 | 
| 10 cc. bromine water. 
