Can we form any idea why the crystals build themselves up so
systematically? Dr. Tyndall says we can, and I hope by the help
of these small bar magnets to show you how he explains it. These
little pieces of steel, which I hope you can see lying
on this white cardboard, have been rubbed along a magnet until
they have become magnets themselves, and I can attract and lift
up a needle with any one of them. But if I try to lift one bar
with another, I can only do it by bringing certain ends together.
I have tied a piece of red cotton (c, Fig. 21) round one end of
each of the magnets, and if I bring two red ends together they
will not cling together but roll apart. If, on the contrary, I
put a red end against an end where there is not cotton, then the
two bars cling together. This is because every magnet has two
poles or points which are exactly opposite in character, and to
distinguish them one is called the positive pole and the other
the negative pole. Now when I bring two red ends, that is, two
positive poles together, they drive each other away. See! the
magnet I am not holding runs away from the other. But if I bring
a red end and a black end, that is, a positive and a negative end
together, then they are attracted and cling. I will make a
triangle (A, Fig. 21) in which a black end and a red end always
come together, and you see the triangle holds together. But now if
I take off the lower bar and turn it (B, Fig. 21) so that two red
ends and two black ends come together, then this bar actually
rolls back from the others down the cardboard. If I were to break
these bars into a thousand pieces, each piece would still have two
poles, and if they were scattered about near each other in such a
way that they were quite free to move, they would arrange
themselves always so two different poles came together.