It holds in every case, so long as the particles which make
the medium turbid are small enough. And please to recollect that it
matters not in the least whether the medium which is rendered turbid is
solid, liquid, or air. Sulphur is yellow in mass, and mastic varnish is
nearly white, while tobacco smoke when condensed is black, and very
minute particles of water are colorless; it matters not what the color
is, the loss of light is _always_ the same. The result is simply due to
the scattering of light by fine particles, such particles being small in
dimensions compared with a wave of light. Now, in this trough is
suspended 1/1000 of a cubic inch of mastic varnish, and the water in it
measures about 100 cubic inches, or is 100,000 times more in bulk than
the varnish. Under a microscope of ordinary power it is impossible to
distinguish any particles of varnish; it looks like a homogeneous fluid,
though we know that mastic will not dissolve in water.
Now a wave length in the red is about 1/40000 of an inch, and a little
calculation will show that these particles are well within the necessary
limits. Prof. Tyndall has delighted audiences here with an exposition of
the effect of the scattering of light by small particles in the formation
of artificial skies, and it would be superfluous for me to enter more
into that.
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