A DROP OF WATER. 87 
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 pow- 
dered 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 micro- 
scope. 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. 
Can we form any idea why the crystals build them- 
selves 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 
