Page:Popular Science Monthly Volume 21.djvu/386

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374
THE POPULAR SCIENCE MONTHLY.

of steam-boiler, has now very few adherents, since Bunsen has given an explanation of the phenomenon that makes the supposition of a cavity in the interior of the earth unnecessary.

Bunsen gave his sagacious explanation of the periodical eruptions of the Great Geyser after observations and researches which he himself undertook in the year 1846. He found that the temperature in the geyser-tube, R, Fig. 1, is in a state of continuous increase between one eruption and another. Thus, if the whole column of water, R, is divided into layers of a specified thickness, one of these layers, for example, at a certain depth below the surface would show immediately after an eruption a temperature of 187°, which would rise after an interval to 188°, and then to 189°, etc. The more deeply situated layers would have a higher temperature at first, which would increase in the same manner. In no layer, however, would the temperature at which water is changed into steam be indicated before an eruption. The passage of water into steam—that is, its boiling—does not take place under all circumstances at 212°, but only when the pressure on its surface is equal to the weight of one atmosphere, or fifteen pounds to the square inch. If the surface in question is exposed to a lower or a higher pressure than this, water will boil at a correspondingly lower or higher temperature. Thus the boiling-point is depressed as we ascend mountains and enter regions where the pressure of the superincumbent atmosphere is less than fifteen pounds to the square inch. Conditions also exist in nature in which the boiling-point is raised to more than 212°, and may be found, for example, in the intermittent springs. To return to the tube R in the Great Geyser (Fig. 1), the water in which we have divided into a number of horizontal layers: a greater weight is put upon each successive layer in the descending series, since each one has to bear the weight of all the layers above it in addition to that of the atmosphere. Water, the temperature of which exceeds 212°, is called superheated. It has the property of being convertible instantly into steam as soon as the weight laid upon it is removed. Bunsen turned this physical behavior of water to the explanation of the eruptions of the Great Geyser. The water brought up in the streams (W', Fig. 1) is in a superheated condition on account of the depth from which it comes and the pressure to which it is exposed. It is not, however, converted into steam on reaching R, because that is prevented by the weight of the column of water above. The layers of water above it are, however, heated by convection from it, so that they become specifically lighter, and originate the axial current toward the surface. As soon as it reaches the surface, the water is cooled by radiation, and a part of it falls back in the shape of the downward currents along the sides of the tube, causing a depression of temperature, and a corresponding delay in the formation of steam. But these currents are weak, their water is gradually becoming warmer as it gets farther from the surface, so that they only reach about the middle