192 
from the innumerable particles of carbon. Owing to the 
absorption a white surface seen through the column of liquid 
seems of diminished whiteness. Hence, under ordinary 
circumstances, tlie light which comes to the eye has a two- 
fold origin, part being transmitted light and part dispersed 
light. My first object was to dissociate these two pheno- 
mena. I therefore used cylinders which were covered with 
black cloth, admitting light by circular apertures at the 
bottom, 8 mm. in diameter. In this way the dispersed light 
was almost wholly cut off. In some cases a feebly nebulous 
light could be seen around the apertures, but it was very 
slight and did not interfere with experiments to determine 
the absorptive properties. With any attempt to explain by 
ph3^sical optics the analogy of the absorption of light by 
solutions and diffusions I have nothing to do, but on the 
supposition that there was continuity. I was led to expect 
that a function of the same form would express the intensity 
of the transmitted light in both cases. Also independently 
of such considerations it seemed probable from a reflection 
upon the mode of distribution of the particles throughout 
the mass of fluid, that the function would be the same as 
for transparent solutions. In thelatter case, both experiment 
and reasoning from first principles concur in giving a 
formula. 2a k for the intensity of light passing through a 
column t units long. As I have pointed out in another 
paper, this implies an expression of the form 2a/c,^ 
denoting the connection between the transmitted 
light and the quantity of colouring matter. There- 
fore we miglit expect a similar expression to hold 
in the case of diffusions (the term seems to me more 
convenient than to speak of turbid solutions). If such be 
the case, one consequence will be that, if we have two 
cylinders containing in equal bulks of water Q and Q' of 
solid matter in suspension, if we adjust the columns so as 
to obtain the same intensity of light when we regard exter- 
