1333 
the change, then the photo-chemical effect will be ineasured bj a 
definite quantity of H produced from A. This effect, which raay be 
termed E, should always correspond to the same araount of light 
energy. 
The total amount of light or “Light flux” over a time t will be 
equal to It, where I is the intensity. Then the photo-chemical effect 
E should be proportional to this, i.e. E — Kit. 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 conversly. 
It will also be seen that in sunlight the difference bet ween 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 ethylalcohol by bromine in dilute aqueous solution 
has been investigated by Bugarszky (Zeit. Phys. Ohem. (IbOl) 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 3 COH + 2 HBr . . . . (I) 
CH 3 COH -f Br, + H,0 = CH 3 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. 
Quality of light. 
Time of 
exposure. 
Amount of 
bromine 
added 
in grams. 
. 1 
Unchanged 
bromine. 
Volume of solution. 
a) Sunlight 
1 1 
158' 
0.06527 
0.01545 
1 5 C.C. ethyl-alcohol and 
Darkness 
158' i 
Do 
0.C6412 
( 10 c.c bromine water. 
b) Sunlight 
286' 
Do 
0.01084 
10 c.c. ethyl-alcohol and 
Darkness 
j 286' 
Do 
0.04628 
1 10 c.c. bromine water. 
c) Diffused light 
159' 
Do 
0.02895 
5 c.c. ethyl-alcohol and 
Darkness 
159' 
Do 
0.06248 1 
( 10 c.c. bromine water. 
d) Diffused light 
' 285' 
Do 
0.02012 
j 10 c.c. ethyl-alcohal and 
Darkness 
285' 
1 Do 1 
0.04613 j 
j 10 c.c. bromine water. 
