198 
This second series of experiments confirms the result of the 
first series that there is a slight departure from the rule, when 
the strength of one diffusion is several times a multiple of 
the other. Both series show a tendency to make the 
column in the comparison cylinder slightly too long. We 
must not hastily conclude that in these cases the theory is 
not applicable. It seems to me that we should expect such 
a result, for the medium is not perfectly transparent, and 
in one cylinder we have a column of this medium (water) 
several times a multiple of the length of the column in the 
other. This would require some slight compensation. In 
these experiments I have taken the lower level of the 
meniscus as the proper reading. In some of the stronger 
diffusions it was a little difficult to determine this exactly. 
On the whole, I think that the above experiments are 
favourable to the assumption that for a column of fluid 
containing finely divided carbon in suspension the relation- 
ship ^ constant holds, if the intensity of the transmitted 
light remain constant. If we represent the above results 
graphically, and take as the theoretical curve the rectan- 
gular hyperbola cry =25 ‘44, it will be seen that the results 
of the experiments do not depart far from the curve. 
In a paper read at the last meeting of the section, I 
suggested a method for testing the assumed laws of the 
absorption of light. I also have applied the same reasoning 
and method of experiment to carbon diffusions. In one 
series of experiments I took a diffusion containing 1*934 
cub. c. in 500 cub. c. The standard shades of grey used 
were one consisting of 10 grms. BaS 04 and 0*012 of lamp- 
black — this I denote by Wa — the other consisted of 10 grms. 
of BaS 04 iJ^nd 0 0 48 of lamp-black — this I denote by Wb. 
The materials were well incorporated by shaking and 
grinding. To the powder I then added a little water so as 
to obtain a mixture of suitable consistence to be used as 
a paint. This was applied by a brush to pieces of cardboard 
