Krug von Nidda on the Mineral Springs of Iceland. 99 



2. Those that are only agitated at particular periods, and are 

 perfectly tranquil during the remaining time(intermitt'uig 

 thermal springs). 



3. Those whose surface is always tranquil, and which are 

 never in an agitated or boiling condition. 



The Icelanders make a similar division, for they term the 

 springs of the first and second classes ffuerer 9 that is spouting 

 springs ; and those of the third class Laugar, that is warm-baths. 

 The springs of the first class have always, at the surface, a tem- 

 perature equal to that of boiling water under the simple pres- 

 sure of the atmosphere. The springs of the second class ac- 

 quire their boiling heat only during their sudden ebullition, and 

 have their temperature considerably lowered, during their pe- 

 riods of repose. The springs of the third class never reach the 

 boiling point. 



It is probable that the thermal springs acquire their elevated 

 temperature from the mass of steam which streams from the sub- 

 terranean warm spring through the column of water. If the 

 steam could always pass freely through the aqueous column, the 

 layers of water must always preserve equably the temperature 

 indicated by the boiling heat, under that pressure, to which each 

 particular layer of water is subjected ; at the surface this would 

 of course be 80 R. (212 F.). If, on the contrary, the masses 

 of steam are interrupted by numerous canals during their pas- 

 sage to the surface ; if, for example, they are enclosed in hollow 

 spaces ; the temperature of the upper layers of water must sink, 

 while owing to the continued evaporation caused by the atmo- 

 sphere, a large quantity of heat is lost, which is not replaced 

 from beneath. But a circulation of the warm and cold layers of 

 water, according to their specific gravity, seems to be rendered 

 difficult, by the narrowness and the complicated windings of the 

 pipe. 



It is doubtless, on such hollow spaces, that the simple me- 

 chanism of intermitting springs depends. In them the evolved 

 masses of steam will be detained by the column of water which 

 closes up the canal leading upwards to the mouth ; these masses 

 thus necessarily accumulate, and force the water always deeper 

 downwards into the cavity, until at last their expansive power 

 becomes so great, that they open the canal communicating with 

 the external mouth, escape with violence through the column 



