LE ROY C. COOLEY. 81 
layer instead of being uniformly distributed through the 
volume of solution. Let us conceive the whole number 
in the volume Z’ to be arranged in that way, they would 
form a layer whose area would be Z?. The number of 
particles along one side of this square would be repre- 
sented by 5 and hence the whole number in the layer 
would be — 
But since color is due to the absorption of luminous 
energy from white light, and since the absorptive om 
of a given mass is constant, this number of particles (a =) 
would absorb the same amount of energy and affect the 
eye in the same degree whether arranged in asingle lay- 
er, whose area is Z’, or distributed uniformly through a 
volume Z’, the area of whose section exposed to the eye 
is /’, the same as before. Hence = represents the small- 
est number of particles of the coloring matter which, 
uniformly distributed in the volume 2’, is required to af- 
fect the eye. 
At this point we can bring our reasoning to the test of 
experiment. If the same number of particles will yield 
the same intensity of color whether arranged in a single 
layer with area /* or distributed through a volume 2’, we 
must be willing to admit that a given mass of coloring 
matter will produce the same intensity of color whether 
the volume of the solution be large or small, the only 
conditions being that the area of the solution exposed 
to the eye shall be unchanged, and the cross section of 
the vessel which contains it shall be uniform. 
To test this by experiment, two exactly similar vessels 
were constructed. Each was eight inches high, with a 
cross section inside of two inches square. Their walls 
were of wood, with their inside surfaces painted dead 
black, while their bottoms were coiorless plate glass. 
These vessels were supported, side by side, a little above 
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