90 THE FAIRY-LAND OF SCIENCE. 
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. 
Natural diamonds are crystals of carbon, made inside 
the earth.* Rock-crystals, which you know probably 
under the name of Cape May or California diamonds, 
are crystallized quartz; and so, with slightly different 
colourings, are agates, opals, jasper, cairngorms, and 
many other precious stones. Iron, copper, gold, and 
sulphur, when melted and cooled slowly build them- 
selves into crystals, each of their own peculiar form, 
and we see that there is here a wonderful order, such 
as we should never have dreamed 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 regu- 
larly. 
Can we form any idea why the crystals build them- 
* It is possible to make diamonds artificially, but they are 
very small. 
