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Experience in the laboratory brings under our notice cases 
where matter is so finely divided as not to be separable 
from fluids by filtration^ and showing but slight tendency 
to settle as a precipitate, and in some cases we have liquids 
which are apparently transparent, and yet are considered to 
hold solid matter in suspension. May not such examples 
be intermediate between solutions and cases where extremely 
fine particles are uniformly diffused through some trans- 
parent medium ? If then we consider the passage from 
tranparency to turbidity as a continuous one, and if for a 
transparent fluid we have established some law of absorption 
of light, may not the same law be applicable to a turbid 
solution ? The subject seemed to me interesting both as a 
scientific enquiry and on account of its application to quan- 
titative analysis. 
Suppose we have diffused through a liquid some finely 
divided solid matter; the action of such a turbid solution 
on light will be twofold, it disperses light and it absorbs 
light. By reason of the first action we are made aware of 
the colour of the turbidity; by reason of the second, any 
object seen through the liquid seems of diminished dis- 
tinctness. 
As a typical case, take carbon diffused through water. In 
a paper which I read at the last meeting of the Physical 
and Mathematical Section, I alluded to some attempts to 
obtain a soluble black, in order to make some experiments 
on the absorption of light. To assist my judgment as to 
the appearance which such a liquid should present, I had a 
cylinder containing a little carbon diffused through water. 
Weak diffusions of carbon in the colorimeter gave the same 
appearance when I looked at external white surfaces as I 
should have expected a liquid containing a soluble black in 
solution to give under the same conditions. A diffusion of 
carbon both disperses light and absorbs light. The disper- 
sion gives rise to a greyish tint, due to the light coming 
