16 
and for t units a{l<f)\ Finally, if we suppose that 
there are various kinds of light, we have 
T = 
As a probable expression for the intensity of light passing 
through a column t units long and containing q units of 
colouring matter per unit of length. I think that in many 
cases where the relation qt^ constant fails, it may be 
traced to some decomposition having taken place, or to some 
change effected by light ; for example, I commenced some 
experiments with ferricyanide of potassium, but as it did 
not prove a suitable salt for making experiments without 
some special precautions with regard to the action of light, 
I discontinued them. As I am not aware that any one has 
particularly noticed this darkening, a few remarks may be 
interesting. A standard solution was prepared containing 
0*8 grins, in 500 cub. c. In the afternoon having occasion 
to use this standard for comparison with another, the result 
was not satisfactory owing to its transparency not being so 
perfect as when freshly made. On the following morning 
I made a fresh standard solution of the same composition ; it 
differed from the old in being more transparent, and I 
thought that it had more of a greenish tint. This new 
solution being left on the table before the window, after a 
time became of diminished transparency ; also on looking 
down into the cylinder a very faint red cloudiness was 
perceptible. I also compared a solution containing 3*2 grms. 
in 500 cub. c. which had been freshly prepared with a 
solution containing 6*4 grms in 500 cub. c. this solution had 
been prepared on the previous day and had been exposed to 
light during that interval. I found the length of the 
column indicated by theory decidedly too great; it occurred 
