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needle. I now reverse the poker, bringing its knob under-
most; the knob is now a north pole and attracts the south
end of a magnetic needle. Get such a poker and carefully
repeat this experiment; satisfy yourselves that the fluids
shift their position according to the manner in which the
poker is presented to the earth. It has already been stated
that the softest iron possesses a certain amount of coercive
force. The earth, at this moment, finds in this force an an-
tagonist which opposes the full decomposition of the neu-
tral fluid. The component fluids may be figured as meet-
ing an amount of friction, or possessing an amount of ad-
hesion, which prevents them from gliding over the atoms
of the poker. Can we assist the earth in this case ? If we
wish to remove the residue of a powder from the interior
surface of a glass to which the powder clings, we invert
the glass, tap it, loosen the hold of the powder, and thus
enable the force of gravity to pull it down. So also by
tapping the end of the poker we loosen the adhesion of
the fluid to the atoms and enable the earth to pull them
apart. But, what is the consequence? The portion of
fluid which has been thus forcibly dragged over the atoms
refuses to return when the poker has been removed
from the line of dip; the iron, as you see, has become
a permanent magnet. By reversing its position and
tapping it again we reverse its magnetism. A thoughtful
and competent teacher will well know how to place these
remarkable facts before his pupils in a manner which will
excite their interest; he will know, and if not, will try to
learn, how, by the use of sensible images, more or less gross,
to give those he teaches definite conceptions, purifying
these conceptions more and more as the minds of his pupils
become more capable of abstraction. He will cause his
logic to run like a line of light through these images, and
by thus acting he will cause his boys to march at his
side with a profit and a joy, which the mere exhibition of

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