52 Prof. Bischof on the Natural History of 



limes found. Thus Forster* asserts, that in the neighbourhood 

 of Tanna, a volcano on one of the Neio Hebrides, the hot springs 

 vary several degrees in temperature from one day to the other. 

 There is not, perhaps, a more striking example of the inti- 

 mate connexion existing between volcanic phenomena and hot 

 springs than in Iceland. As the volcanic eruptions are there 

 confined to the district of the trachyte- formation, so also are the 

 principal mineral springs only found in this formation ;t from 

 which it seems natural to infer, that it is one and the same pro- 

 cess acting in both cases, but in a different manner.J 



The hot springs in this volcanic island confirm Krug Von 

 Nidda'*s system of classing thermal springs — namely, 1. such 

 as are constantly bubbling and boiling up — permanent ther- 

 mals ; 2. those in which this ebullition only takes ^lace at par- 

 iicular periods, and which are perfectly tranquil during the re- 

 maining time — intermitting thermals ; and, i], those whose sur- 

 face is always undisturbed, and in whiclt no bubbling or boiling 

 ever takes place. The springs of the first class always have a 

 temperature at the surface equal to that of boiling water under 

 the usual atmospheric pressure. Those of the second class only 

 reach the boiling point during their temporary ebullition, and 

 lose considerably in temperature during their period of rest. 

 The springs of the third class never reach the boiling point of 

 water. 



The most famous of the intermittent springs is the Great 

 Geyser. At the time when Krug Von Nidda visited it, it pre- 

 sented two different kinds of eruption. The smaller ones were 

 repeated regularly every two hours ; and the water was thrown 

 only from fifteen to twenty feet high. The greater ones suc- 

 ceeded each other at intervals of from twenty-four to thirty 

 hours ; in these cases, the masses of steam ascended to the 

 clouds, and the water spouted to a height of ninety feet. For 

 two hours after one of the smaller eruptions, during which 

 time there were no traces of action, and only thin clouds of 



* Journ. de Phys. 1779, p. 434. 



t All the hot springs of Mexico also rise out of trachyte and dolerite rocks' 

 Burkart, p. 363. 



X Krug Von Nidda on the mineial springs of Ireland, p. 272, in Karston"; 

 Archiv. t. ix. p. 247, and in Jameson's Phil. Journal, vol. xxii. p. 90 and 220 j 



