Nearly all the metals are characterized by the crystals, which are formed as they pass from a state of intense heat to that of comparative coldness. It is by this process they have been formed when in the mine or vein in the rocks. The earth was once a fiery mass of molten matter, as seen even now when a volcano is in a state of eruption. And it was only by the cooling of the outside shell of the earth, or crust, as it is called, that it became habitable.

When the crust was cooling down the metals crystallized among the cooling rocks and gradually formed the crude arts. You may represent by a very pretty experiment the manner in which this cooling off of the earth took place. Obtain a little flour of sulphur and put it in a red earthenware unglazed jar. Thrust it well into the fire and watch the rust. As soon as the heat has penetrated the vessel the yellow powdery sulphur becomes first of all brown, and then assumes the consistency of thick birdlime. Take out a little of this on the end of a stick and plunge it into cold water. It can then be pulled backwards and forwards like cobblers’ wax. This well represents the state of the half-cooled crust of the earth.

Meanwhile the sulphur on the fire begins to boil, and looks very much like bubbling treacle. Remove it from the fire and allow it to cool. When quite cool the surface will be a flat, yellow mass, like ordinary roll sulphur, which, when ground, give the ordinary flour of sulphur.

With a sharp knife separate the mass from the vessel and look at the under-surface. There it will be found to have assumed a very different form, owing to the exclusion of the air, and consequent slower cooling. Large six-sided crystals, transparent, and of a most exquisitely delicate yellow, will be seen, piled on one another as appear the masses of ore in rocks.

Nature always works in such cases on such a gigantic scale that it seems at first difficult to believe that such huge piles as the Giant’s Causeway in Ireland, or Fingals in Scotland, or the lodes of tin ore in Cornwall, worked by the Phoenicians three thousand years ago, and still being worked, were all formed by the same process.

The time that the earth must have taken to cool fairly staggers the imagination, yet it is only from guessing, by means of such a study as this, that geologists are able to form any idea of how long ago it was that the earth’s crust became cool enough to allow animal and plant life to exist upon it.

The most beautiful crystalline form is perhaps the diamond, and yet this precious gem is but the same thing, chemically, as charcoal. Charcoal is pure carbon in the uncrystallized state, which the magic of crystallization has transformed into the symbol of all that is brilliant and beautiful.