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sentant of what occurs in Iceland. Imagine the case of a simple thermal siliceous spring, whose waters trickle down a gentle incline; the water thus exposed evaporates, and silica is deposited. This deposit gradually elevates the side over which the water passes, until, finally, the latter has to take another course. The process is repeated here, the ground being elevated as before, and the spring has again to move forward. Thus it is compelled to travel round and round, depositing its silica and deepening the shaft in which it dwells, until finally, in the course of ages, the simple spring has produced that wonderful apparatus which so long puzzled and astonished both the tourist and the philosopher.
Previous to an eruption, both the tube and basin are filled with hot water: detonations which shake the ground are heard at intervals, and each explosion is succeeded by a violent agitation of the water in the basin. The water column is lifted up, forming an eminence in the middle of the basin, and an overflow is the consequence. These detonations are evidently due to the production of steam in the ducts which feed the geyser tube, which steam, rushing into the cooler water of the tube, is there suddenly condensed, and produces the noise. In 1846 Professor Bunsen succeeded in determining, a few minutes before a great eruption, the temperature of the geyser tube, from top to bottom; and these observations revealed the extraordinary fact, that at no part of the tube did the water reach its boiling point.
In the annexed sketch [fig. 1] I have given, on one side, the temperatures actually observed, and on the other side the temperatures at which water would boil, taking into account the pressure of the atmosphere and of the superincumbent column of water. The nearest approach to the boiling point is at A, thirty feet from the bottom; but even here the water is 2° Centigrade, or more than 31° Fahr., below the temperature at which it could boil. How then is it possible that an eruption could occur under such circumstances?
Fix your attention upon the water at the point A, where the temperature is within 2° C. of the boiling point. Call to mind the lifting of the column when the detonations are heard. Let us suppose that by the entrance of steam from the ducts near the bottom of the tube, the geyser column is elevated six
feet, a height quite within the limits of actual observation; the water at A is thereby transferred to B. Its boiling point at A is 123.8°, and its actual temperature 121.8°; but at B its boiling point is only 120.8°; hence, when transferred from A to B, the heat which it possesses is in excess of that necessary to make it boil. This excess of heat is instantly applied to the generation of steam: the column is lifted higher, and the water below is further relieved. More steam is generated, and from the middle downwards the mass suddenly bursts into ebullition. The water above, mixed with steam-clouds, is projected into the atmosphere, and we have the geyser eruption in all its grandeur.
By its contact with the air the water is cooled, falls back into the basin, partially refills the tube, in which it gradually rises, and finally fills the basin as before. Detonations are heard at intervals, and risings of the water in the basin. These are so many futile attempts at an eruption, for not until the water in the tube comes sufficiently near its boiling temperature to make the lifting of the column effective, can we have a true eruption.
To the illustrious Bunsen we owe this beautiful theory, and now let us try to justify it by experiments.1 Here is a tube of galvanized iron, six feet long, A B [fig. 2], surmounted by a basin, C D. It tapers from a diameter of 6 inches at the bottom to a diameter of 11⁄2 inch at the top. It is heated by a fire under
1 The first artificial geyser was, I believe, constructed by the late Dr. Bromeis of Marburg.
neath; and, to imitate as far as possible the condition of the geyser, the tube is encircled by a second fire, F, at a height of two feet from the bottom. Doubtless the high temperature of the water, at the corresponding part of the geyser tube, is due to a local action of the heated rocks. The tube is filled with water, which gradually becomes heated to the boiling temperature; and regularly, every five minutes afterwards, the liquid is ejected into the atmosphere.
There is another famous spring in Iceland called the Strokkur, which is usually forced to explode by stopping its mouth with clods. We can imitate the action of this spring by stopping the mouth of our tube A B-not too tightly be it observed with a cork. The heating progresses. The steam finally attains sufficient tension to eject the cork, and the water, suddenly relieved from the pressure, bursts forth into the atmosphere. The ceiling of this room is nearly thirty feet from the floor, but the eruption has reached the ceiling, from which the water now drips plentifully. In fig.  is given a section of the Strokkur.
By stopping our model geyser tube with corks, through which glass tubes of various lengths and diameters pass, the action of many of the other