If you take a mixture of sulphur and iron filings, and mix them into a paste with water, and then bury them in the ground, after a while they will become hot and send out a great quantity of steam. Some people have imagined that this is the way in which volcanic activity is produced, but no one thinks so now, and for very good reasons, for there is nothing in the action so produced at all capable of accounting for long continued eruptions, or for the flowing out of the lava. Sir Humphrey Davy found out, that nearly all kinds of salt and earth, which had been before looked upon as simple substances, contained certain metals united to oxygen, which, (as you should know,) is one of the parts of water, and, in the form of gas, of the air we breathe. If you put a piece of one of these metals into water, it is in such a hurry to join itself to the oxygen of the water, that you set it on fire. A little piece of _potassium_, (the metal of the salt called _potash_, or _pearlash_,) will float for a few moments on the surface of the water, burning with a purple flame in the prettiest manner imaginable. [Illustration] Sir Humphrey thought he had got the true explanation of the thing. These wonderful metals formed the basis of every substance in lava, and supposing them to exist in the centre of the globe, it was only necessary for water to get to them to set them on fire, and thus to give rise to earthquakes and volcanoes. It appeared only to be required that a crack should be made at the bottom of the sea, and the water flow into the beds of metal underneath, and the violent action produced would burst through the surface, and throw out streams of melted matter, accompanied with great clouds of steam, just as takes place in the eruptions of Mount Vesuvius and other volcanoes. But you would find it very difficult to think how this can account for continual and nearly steady operations, such as have been known to exist in the Geysers of Iceland for more than 1000 years, and in Mount Stromboli for much longer than that, so we will try another guess. It has been observed in descending deep mines, that the earth gets warmer the deeper you go. The usual mode of trying it is to dig little holes, as you go down, in the sides and bottom of the mine, and put a thermometer into them. A great many observations of this kind, have been made in different parts of the world, and the greatest care has been taken to allow for all causes of irregularity, consisting in climate and local peculiarities. Comparing these together, it has been determined that the temperature increases about one degree every fifteen feet of descent. A similar conclusion has been drawn from the heat of the water of the Artesian Wells. But, perhaps, you do not know what Artesian Wells are, so I will tell you.