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 him- 

 self 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 lay- 

 ers 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 su- 

 perincumbent 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 inter- 

 mittent 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 lay- 

 ers: a greater weight is put upon each successive layer in the descend- 

 ing 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 expla- 

 nation 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 tempera- 

 ture, 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 



