64 Dr Daubeny on Thermal Springs, 



this kind can be expected to be derived, since it is probable, that 

 these are merely obtained from the strata through which the wa- 

 ter percolates. In the case of volcanos, indeed, the carbonate 

 and muriate of soda so generally carried by sublimation into the 

 different vents through which the vapours issue, may, with much 

 reason, be referred to the seat of the igneous action itself, and 

 the occurrence of these salts may thus be regarded as a pre- 

 sumption in favour of the iheory, which assumes that sea-water 

 has some share in the effects produced. 



But, in the case of hot springs, we cannot be sure that the 

 rocks themselves may not have furnished these ingredients, know- 

 ing as we do, that common salt is present every where, and that 

 water, impregnated with carbonic acid gas, is a ready solvent of 

 the alkali which felspathic rocks may contain. It is curious, in- 

 deed, that, in the Pyrenees, the warm springs appear to contain 

 soda uncombined with carbonic acid, for I found that hme was 

 not precipitated from its aqueous solution, when added to the 

 water of Barege fresh drawn, and that even barytic water re- 

 mained unaffected till some moments had elapsed after its addi- 

 tion. Nevertheless, it is possible that the water itself, at a high 

 temperature, assisted by great pressure, may possess a solvent 

 power over the materials of the felspar, and that the mineral al- 

 kali, as well as the silica, which such waters contain, may be 

 derived from this source. 



It is to the gases, therefore, accompanying hot springs, that 

 we ought chiefly to look, as affording us a clew to the cause of 

 their greater heat, and to those especially which are most abun- 

 dantly and most generally present. Now, it has been already re- 

 marked, that the very same aeriform fluids which appear during 

 the more languid states of volcanic action, are also evolved by 

 hot springs ; thus, as we have seen in the Pyrenees, sulphuret- 

 ted hydrogen is a very common ingredient in them, and car- 

 bonic acid is even more generally present. 



These two gases, however, will not assist us greatly towards 

 the explanation of the primary cause of their heat ; the former 

 being too often absent to be regarded as essential ; the latter 

 being simply accounted for by the operation of the heat upon 

 calcareous beds. 



