sees in one place a great pool full of black bubbling contents, so hot that an egg thrown in the spring will be boiled in a minute or two; there he sees another spring throwing up boiling mud a few feet in the air; there another one, quiet now, but which may at any time burst out and send its hot contents high above the heads of the spectators; here a great hole in the ground, out of which constantly issues a column of steam, and everywhere are cracks and crevices in the earth, out of which come little jets of steam, and which give the idea that it would not require a very heavy blow to break in, at any point, the crust of the earth, and let the adventurous traveller drop down into the boiling mass below.

In Iceland the Geysers are not quite so terrible in their aspect as those in California, but they are bad enough.  Their contents are generally water, some hot and bubbling, and some hot and still; while the Great Geyser, the grandest work of the kind in the world, bursts forth at times with great violence, sending jets of hot water hundreds of feet into the air.

These wonderful hot springs, wherever they have been found, have excited the greatest attention and interest, in travellers and scientific men, and their workings have been explained somewhat in this way:—­

Water having gradually accumulated in vast underground crevices and cavities, is heated by the fires, which, in volcanic regions, are not very far from the surface of the earth.  If there is a channel or tube from the reservoir to the surface, the water will expand and rise until it fills the basin which is generally found at the mouth of hot springs.  But the water beneath, being still further heated, will be changed into steam, which will at times burst out with great force, carrying with it a column of water high into the air.  When this water falls back into the basin it is much cooler, on account of its contact with the air, and it cools the water in the basin, and also condenses the steam in the tube or channel leading from the reservoir.  The spring is then quiet until enough steam is again formed to cause another eruption.  A celebrated German chemist named Bunsen constructed an apparatus for the purpose of showing the operations of Geysers.  Here it is.

[Illustration:  THE ARTIFICIAL GEYSER.]

You see that the two fires in the engraving—­one lower and larger than the other, because the heat of the earth increases as we get farther from the surface—­will heat the water in the iron tube very much as water is heated in a real Geyser; and when steam enough is formed, a column of hot water is thrown out of the basin.  The great subterranean reservoir is not imitated in this apparatus, but the action is the same as if the tube arose from an iron vessel.  There is a great deal in Bunsen’s description of this contrivance, in regard to the difference in the temperature of the water in that part of the tube between the two fires, and that in the upper portion, which explains the intermittent character of the eruptions of a Geyser, but it is not necessary for us to go into all his details.