110 



IGNEOUS AGENCIES. 



iitrW 



mation of steam, and the phenomena of an eruption. But it is ex- 

 tremely unlikely that this condition should exist in all geysers ; neither 

 is it at all necessary in order to explain the phenomenon of an eruption. 

 To prove beyond question the truth of this theory, Bunsen con- 

 structed an artificial geyser. The apparatus (Fig. 94) consisted of a 

 tube of tinned sheet-iron about ten feet long, expanded into a dish 

 above for catching the erupted water. It may or may not be expanded 

 below for the convenience of heating. It was heated, 

 also, a little below the middle, by an encircling char- 

 coal chauffer, to represent the point of nearest ap- 

 proach to the boiling-point in the geyser -tube. 

 TUien this apparatus was heated at the two points, 

 as shown in the figure, the phenomena of geyser- 

 eruption were completely reproduced : first, the vio- 

 lent explosive simmering, then the overflow, then the 

 eruption, and then the state of quiescence. In Bun- 

 sen's experiment, the eruptions occurred about every 

 thirty minutes. 



Bunsen's Theory of Geyser-Formation. — Accord- 

 ing to Bunsen, a geyser does not find a cave, or even 

 a perpendicular tube, ready made, but, like volcanoes, 

 makes its own tube. Fig. 95 is an ideal section of a 

 geyser-mound, showing the manner in which, accord- 

 ing to this view, it is formed. The irregular line, 

 b a c, is the original surface, and a the position of a 

 hot spring. If the spring be not alkaline, it will 

 remain an ordinary hot spring ; but, if it be alka- 

 line, it will hold silica in solution, and the silica will 

 be deposited about the spring. Thus the mound 

 and tube are gradually built up. For a long time the 

 spring will not be eruptive, for the circulation will 

 maintain a nearly equal temperature in every part 

 of the tube — it may be a boiling, but not an eruptive 

 spring. But, as the tube becomes longer, and the circulation more and 

 more impeded, the difference of temperature between the upper and 

 lower parts of the tube becomes greater and greater, until, finally, the 

 boiling-point is reached below, while the water above is comparatively 

 cool. Then the eruption commences. Finally, from the gradual fail- 

 ure of the subterranean heat, or from the increasing length of the tube 

 repressing the formation of steam, the eruptions gradually cease. Bun- 

 sen found geysers in different stages of development — some playful 

 springs without tubes ; some with short tubes, not yet eruptive ; some 

 with long tubes, violently eruptive ; some becoming old and indisposed 

 to erupt unless angered by throwing stones down the throat. 



Fig. 



94.— Artificial 

 Geyser. 



