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scope, may, when examined by the method alluded to, pro-
duce not only sensible but striking eifects upon the eye.

I take, for instance, this bottle of water intended to
quench your lecturer’s thirst. In the track of the beam it
simply reveals itself as dirty water. So you see that we
are invaded with dirt not only in the air we breathe, but
in the water we drink. And this water is no worse than
the other London waters. Thanks to the kindness of Pro-
fessor Frankland, I have been furnished with specimens of
the water of eight London companies. They are all laden
with impurities mechanically suspended. But you will ask
whether filtering will not remove the suspended matter?
The grosser matter, undoubtedly, but not the more finely-
divided matter. Here is water which has been passed four
times through a filter of bibulous paper, but it is still laden
with fine matter. Here, also, is a bottle kindly sent me by
Mr. Lipscomb, and passed once through his charcoal filter.
But the track of the beam through it is more luminous
than through air, because the quantity of matter suspended
in theiwater is greater than that suspended in air. Here
is another specimen courteously sent to me by the Silicated
Carbon Company. All the grosser matter has been re-
moved, but it is thick with fine matter. N ine-tenths of the
light scattered by these particles is perfectly polarized in a
direction at right angles to the beam, and this release of
the particles from the ordinary law of polarization is a
demonstration of their smallness. I should say by far the
greater number of the particles concerned in this scattering
are wholly beyond the range of the microscope, and no or-
dinary filter can intercept them. There is an aesthetic
pleasure in the drinking of a glass of cold sparkling water,
and I fear these experiments will destroy this pleasure if
you ever enjoyed it. And it is next to impossible by arti-
ficial means to produce pure water. Mr. Hartley, for ex-
ample, some time ago distilled water while it was sur-

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