GEYSER (Icelandic, geysa, to burst forth violently, allied to Eng. gush) is a term applied to the eruptive thermal springs and wells which are found in various parts of the earth's surface in evident connection with the volcanic forces at work below. The geysers in the Yellowstone (q.v.) region are probably the most wonderful of all, but the best known group is in Iceland, about 70 m. from Reikiavik, 16 m. n. of Skalholt, and within sight of the volcano of Flecla. On the slope of a low trap-hill, overlooking the wide grassy valley of the Whitae, or White river, a space of ground measuring perhaps half a mile each way is thickly interspersed with boiling or hot springs, of various sizes, from jets not gr&tter than an overboiling tea-kettle, up to great caldrons, besides ves tiges of others no longer in operation. All are surrounded by silicious incrustations, formed in the course of time by the minute charge of silica infused into the water. The chief apertures are two, respectively called the Great Geyser and the.Stroker (i.e. churn), which are little more than 100 yards apart. The latter is an irregular aperture of from 6 to 8 ft. diameter, down which one may in general safely look, when he sees the water noisily working in a narrower passage about 2.0 ft. below. If, by throwing in a sufli dent quantity of turf, he can temporarily choke this gullet, the water will in a few minutes overcome the resistance, and, so to speak. perform an eruption with magnifi cent effect, bursting up 60 ft. into the air, brown with the turf that has been infused into it, and diffusing steam in vast volumes around.
The appearance of the Great Geyser is considerably different. On the summit of a mount which rises about 15 ft. above the surrounding ground, is a circular pool or cup of hot water, 72 ft. across at its greatest diameter, and about 4 ft. deep, being entirely formed of silicious crust of a dull gray color. At the edge, this water has been found to be 188° F.; in the center, it is considerably higher. From the center descends a pit of 8 ft. width, and 83 ft. deep, up which a stream of highly heated water is continually but slowly ascending, the surplus finding its way out by a small channel in the edge of the cup, and trickling down the exterior of the crusty eminence. Every few hours, the water, with a rumbling noise, rises tumultuously through the pit, and jets for a few feet above the surface of the pool; by and by it subsides, and all is quiet again. Once a day, however, or thereabouts, this tumult ends in a terrific paroxysm, which lasts perhaps a quarter of an hour, and during which the water is thrown in repeated jets from 60 to 80 ft. high, mingled with such volumes of steam as obscure the country for half a mile round. If a visitor be tolerably near on the windward-side, be may catch glimpses of this grand spectacle—the eruption of a water-volcano, it may be termed— and he must needs be charmed with the beautiful jets as they curve outwards and fall, as well as impressed by the sublimity of the whole scene. When quiet is restored, the chalice, and perhaps 20 ft. of the pit, are found empty, and the visitor obtains, so far, a sight of the internal arrangements and structure of the geyser. In a little time, the
water re-ascends to its usuarievel, and there remains for the next day or two, with only those minor disturbances which have been described.
The thermal wells and springs of Iceland may be said to be of three classes—I. Those of continual and uniform ebullition; 2. Those which, while not constantly ebullient, are liable to occasional eruptions; and, 3. Certain wells not yet particularized, which contain tranquil tepid water, but are supposed (at least in some instances) to have formerly been eruptive. It is only in regard to the second class that there is any room for doubt or speculation. To what are we to attribute the occasional eruptions? The theory started by sir George Mackenzie, who visited Iceland in 1810, is, that steam is gathered in some cavernous recess connected with the subterranean channels through which the water rises; and that, when it has accumulated there till such time as the pressure overcomes the* resistance, it bursts forth through the tube, carrying the water before it, and tossing it high into the air. This. mechanical theory, as it may be called, has lost grotind since the announcement of a chemical one by prof. Bunsen, who spent eleven days besides the Great Geyser in 1846. The learned German looks for an explanation of the phenomena to the molecular changes which take place in water after being long subjected to heat. "In these circumstances,* water loses much of the air contained in it; the cohesion of its molecules is greatly increased, and a higher tem perature is required to boil it. When water in this state is brought to the boil, the pro duction of vapor is so instantaneous and so considerable as to cause an explosion. It has been found that the water of the Great Geyser at the bottom of the tube has a tem perature higher than that of boiling water, and this goes on increasing till an eruption ' takes place, immediately before which, it has been found as high as 261° F. This pecu liarity—for so it is, seeing that, in ordinary circumstances, the hotter water at the btt tom would rise to the top till all was equally warm—shows that the heating of the water • in the geyser takes place under extraordinary circumstances. As far as I understand prof. Bunsen, he implies that the great pressure of the column above, and perhaps some mechanical impediments to free circulation in the form of the geyser, gives these required circumstances. Such being assumedly the case, there is an increase in the cohesion of the molecules of the water constantly going on at the bottom, at the same time that the heat is constantly increasing; at length, the latter force overcomes the former—ebullition takes place—an immense volume of vapor is instantaneously engen dered, and an eruption is the consequence." We have to consider this theory in an unusually curious light in connection with a small double geyser, as it may be called, which exists in the group at Reikholt, and in which each pool makes an eruption every few minutes, the other being at those times pacific.