ON ITS EXTERIOR. THERMAL SPRINGS. 201 



situated at the foot of the Byarnefell, has less water 

 than the Great Greysir. The siliceous margin of its 

 basin is only a few inches high and wide. Its eruptions 

 are more frequent than those of the Greysir, but do not 

 announce themselves by subterranean noise. In the 

 Strokr the temperature at the time of eruption is from 

 235-4 to 239 at a depth of 42 feet, and almost 

 212 at the surface. The eruptions of the intermitting 

 boiling fountains, and the small variations in the type of 

 their phsenomena, are quite independent of the eruptions 

 of Hecla, and were in no ways interrupted by those 

 which took place in 1845 and 1846.( 274 ) Bunsen, with 

 his peculiar sagacity both in observation and discussion, 

 has refuted the formerly received hypotheses respecting 

 the periodicity of the Greysir eruptions (subterranean 

 hollows filled as steam-boilers alternately with vapours 

 and with water). According to his view, the eruptions 

 take place because a part of a column of water, which at 

 some lower depth has gained a high degree of tempera- 

 ture under great pressure by accumulated vapours, is 

 pushed upwards, and thus comes under a pressure in 

 which a portion of the water is converted into steam. 

 Thus, " the Geysirs are natural collectors of steam- 

 power." 



Among hot springs, some few are almost absolutely 

 pure, others contain solutions of as many as from 8 to 

 12 solid or gaseous substances. To the first class belong 

 the Luxueil, Pfeffers, and Gastein waters; of which the 

 nature of the efficacy appears so perplexing on account 

 of their purity. ( 275 ) As all springs are principally fed 

 by meteoric water, (water which has fallen in rain, hail, 

 or snow,) they contain nitrogen ; as Boussingault has 



