an antagonist which opposes the
decomposition of the neutral fluid, The component fluids may be
figured as meeting an amount of friction, or possessing an amount of
adhesion, which prevents them from gliding over the molecules 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
magnetic fluids to the molecules 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 molecules 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 know how to place these remarkable facts before his pupils in a
manner which will excite their interest. By the use of sensible
images, more or less gross, he will first give those whom he teaches
definite conceptions, purifying these conceptions afterwards, as the
minds of his pupils become more capable of abstraction. By thus
giving them a distinct substratum for their reasonings, he will confer
upon his pupils a profit and a joy which the mere exhibition of facts
without principles, or the appeal to the bodily senses and the power
of memory alone, could never inspire.

*****

As an expansion of the note on magnetic fluids, the following extract
may find a place here: 'It is well known that a voltaic current
exerts an attractive force upon a second current, flowing in the same
direction; and that when the directions are opposed to each other the
force exerted is a repulsive one. By coiling wires into spirals,
Ampere was enabled to make them produce all the phenomena of
attraction and repulsion exhibited by magnets, and from this it was
but a step to his celebrated theory of molecular currents. He
supposed the molecules of a magnetic body to be surrounded by such
currents, which, however, in the natural state of the body mutually
neutralised each other, on account of their confused grouping. The
act of magnetisation he supposed to consist in setting these molecular
currents parallel to each other; and, starting from this principle,