17 
to me that the discrepancy was due to some action of light 
on the ferricyanide. About six o’clock in the afternoon I 
again compared these solutions; the theoretical length gave 
a colour which was still too dark, but the disparity of 
colour was not so marked as at first. The comparison was 
also disturbed a little by the slightly diminished trans- 
parency of the weaker solution. 
I now prepared a fresh solution, containing 6 ’4 grams in 
500 cub. c., thinking that when we work with solutions 
which vary gradually in colour we are apt to forget the 
initial condition. This new solution seemed quite different 
from the old one of the same strength. The latter was 
much darker and browner. So great was the difference 
that 9T cm. of the old seemed as dark as 22'5 of the new. 
To find whether the darkening was due to the action of 
light or to some intrinsic cause, I divided the newly made 
solution into two equal columns. One I left on the table 
before the window ; the other I kept in a cylinder which 
was closely invested with black cloth. After the lapse of 
six hours I compared them. The one exposed was so much 
darker that 5 cm. of the exposed solution gave a tint as 
deep as 10'9 cm. of the unexposed. This observation was . 
made on the Saturday. On midday of the following Mon- 
day, when I again compared them, the darkening had 
evidently increased, for 3 cm. of the exposed solution gave 
a tint about as dark as that furnished by 10 9 cm. of the 
unexposed. 
Wishing to ascertain whether keeping in the dark would 
reverse the action of light, on Saturday, May 24th, I took 
a solution containing 6’4 grams in 500 cub. c. The solution 
