we can demagnetise it, or
reverse its magnetism, by properly drawing it along the same pole in
the opposite direction. What, then, is the real nature of this
wondrous change? What is it that takes place among the atoms of the
steel when the substance is magnetised? The question leads us beyond
the region of sense, and into that of imagination. This faculty,
indeed, is the divining-rod of the man of science. Not, however, an
imagination which catches its creations from the air, but one informed
and inspired by facts; capable of seizing firmly on a physical image
as a principle, of discerning its consequences, and of devising means
whereby these forecasts of thought may be brought to an experimental
test. If such a principle be adequate to account for all the
phenomena--if from an assumed cause the observed acts necessarily
follow, we call the assumption a theory, and, once possessing it, we
can not only revive at pleasure facts already known, but we can
predict others which we have never seen. Thus, then, in the
prosecution of physical science, our powers of observation, memory,
imagination, and inference, are all drawn upon. We observe facts and
store them up; the constructive imagination broods upon these
memories, tries to discern their interdependence and weave them to an
organic whole. The theoretic principle flashes or slowly dawns upon
the mind; and then the deductive faculty interposes to carry out the
principle to its logical consequences. A perfect theory gives
dominion over natural facts; and even an assumption which can only
partially stand the test of a comparison with facts, may be of eminent
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 colours; their mixture produces white. Now I ask
you to imagine each of these colours to possess a self-repulsive
power; that red repels red, that green repels green; but that red
attracts green and green attracts red, the attraction of the
dissimilar colours being equal to the repulsion of the similar ones.
Imagine the two colours mixed so as to produce white, and suppose two
strips of wood painted with this white-; what will be their action
upon each other? Suspend one of them