54 Prof. Bischof on the Natural History of 



of Iceland are probably caused by the existence of caverns, in 

 which the vapour is retained by the pressure of the column of 

 water in the channel which leads to the surface. Here this 

 vapour collects, and presses the water in the cavern downwards 

 until its elastic force becomes sufficiently great to effect a pass- 

 age through the colunm of water which confines it. The vio- 

 lent escape of the vapour causes the thunder-like subterranean 

 sound, and the trembling of the earth, Avhich ])recede each 

 eruption. The vapours do not appear at the surface till they 

 have heated the water to their own temperature. When so 

 much vapour has escaped that the expansive force of that 

 which remains has become less than the pressure of the con- 

 fining column of water, tranquillity is restored, and this lasts 

 until such a quantity of vapour is again collected as to produce 

 a fresh eruption. The spouting of the spring is, therefore, re- 

 peated at intervals, depending upon the capacity of the cavern, 

 the height of the column of water, and the heat generated be- 

 low.* 



The two distinct classes of eruption in the Geyser^ which we 

 have already mentioned, seem to be attributable to two diffe- 

 rent cavities. A smaller cavity fills quicker, and, therefore, 

 empties itself more frequently ; a larger one fills slower, empties 

 itself seldomer, but with greater violence. But the playing of 

 the Geyser, the Strokr, and some others, is subject to very great 



■■' The eiiiptions of the Geyser and the Strokr, as observed by Krug Von 

 Nidda, agree exactly with his explanation of the action of the intermitting- 

 springs of Iceland. A thick column of smoke suddenly burst out of the 

 latter, and rose to the clouds. The water was hurled with tei'riiic violence 

 out of the crater, and mixed like a fine mist with the rest of the column to 

 a considerable height. From time to time thin streams of wafer were seen 

 shooting in a vertical or oblique direction through the column of smoke, 

 sometimes rising to a height of a hundred feet and upwards. Large stones, 

 which had been previously thrown in, were flung almost out of sight, and 

 many so perfectly vertically that they fell down again into the crater, and 

 were again throAvn up into the air, like a juggler's ball. The whole of the 

 water was thrown out at the beginning ; and afterwards, the column which 

 ascended from the opening, was composed only of steam, which rushed out 

 ■with a whistling and hissing noise, and rose with incredible velocity into 

 the clouds. It continued for three quarters of an hour in this state of a(!fti- 

 vity. It then again became (j liescent, except that the water, deep in tlu' 

 tube, continued, as usual, to boil violently. 



