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A LECTURE ON MAGNETISM. 367

stand the test of a comparison with facts, may be of emi-
nent use in enabling us to connect and classify groups of
phenomena. The theory of magnetic fluids is of this latter
character, and with it we must now make ourselves familiar.

With the View of stamping the thing more firmly on
your minds, I will make use of a strong and Vivid image.
In Optics, red and green are called complementary colors;
their mixture produces white. Now I ask you to imagine
each of these colors to possess a self-repulsive power; that
red repels red, and that green repels green; but that red
attracts green and green attracts red, the attraction of the
dissimilar colors being equal to the repulsion of the similar
ones. Imagine the two colors mixed so as to produce
white, and suppose two strips of wood painted with this
white; what will be their action upon each other? Sus-
pend one of them freely as we suspended our darning-
needle, and bring the other near it; what will occur ? The
red component of the strip you hold in your hand will re-
pel the red component of your suspended strip, but then it
will attract the green; and the forces being equal they neu-
tralize each other. In fact, the least reflection shows you
that the strips will be as indifferent to each other as two
unmagnetized darning-needles would be under the same
circumstances.

But suppose, instead of mixing the colors, we painted
one half of each strip from centre to end red, and the other
half green, it is perfectly manifest that the two strips would
now behave toward each other exactly as our two magnet-
ized darning-needles—the red end would repel the red and
attract the green, the green would repel the green and at-
tract the red; so that, assuming two colors thus related to
each other, we could by their mixture produce the neutral-
ity of an unmagnetized body, while by their separation we
could produce the duality of action of magnetized bodies.

But you have already anticipated a defect in my con-

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