The particles of nearly all substances, when left
free and not hurried, can build themselves into crystal
forms. If you melt salt in water and then let
all the water evaporate slowly, you will get salt-crystals;
— beautiful cubes of transparent salt
all built on the same pattern. The same is true
of sugar; and if you will look at the spikes of an
ordinary stick of sugar-candy, such as I have here,
you will see the kind of crystals which sugar forms.
You may even pick out such shapes as these from the
common crystallized brown sugar in the sugar basin,
or see them with a magnifying glass on a lump of white
sugar.
But it is not only easily melted substances such as
sugar and salt which form crystals. The beautiful
stalactite grottos are all made of crystals of lime.
Diamonds are crystals of carbon, made inside the earth.
Rock-crystals, which you know probably under the name
of Irish diamonds, are crystallized quartz; and so,
with slightly different colourings, are agates, opals,
jasper, onyx, cairngorms, and many other precious stones.
Iron, copper, gold, and sulphur, when melted and cooled
slowly build themselves into crystals, each of their
own peculiar form, and we see that there is here a
wonderful order, such as we should never have dreamt
of, if we had not proved it. If you possess a
microscope you may watch the growth of crystals yourself
by melting some common powdered nitre in a little
water till you find that no more will melt in it.
Then put a few drops of this water on a warm glass
slide and place it under the microscope. As the
drops dry you will see the long transparent needles
of nitre forming on the glass, and notice how regularly
these crystals grow, not by taking food inside like
living beings, but by adding particle to particle
on the outside evenly and regularly.
Week 12
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.
