106 SPRINGS, RIVERS, CANALS, LAKES, 



large stones, and raise them to the height of 3 or 4 ells, with a 

 noise like the explosion of a great gun. 



From the foregoing history, we may take occasion to reflect a 

 little on the variety of exhalations prepared in and flying out from 

 the vast subterraneous magazines and repositories, as to their qua- 

 lities and effects, some being cold and dry, resembling air or wind, 

 as those near Peroul, and in the caverns of mountains, especially 

 those of ^olus, and other hills of Italy, as also in mines ; others 

 are inflammable, and of a bituminous nature, though not actually 

 warm, as those near \Vigan in Lancashire; there are also many 

 steams very hot, sulphureous, and saline, more especially those in 

 the natural stoves, sweating vaults, grots, baths, and the volcanos 

 near Naples, Bajae, Cuma, and Puzzuolo, as also in some of the 

 subterraneous works at Rome ; others there are of an arsenical and 

 such like noxious qualities, as in the Grotta del Cane, on the bank 

 of the Lago Agnano ; in several mines, and in poisonous springs 

 and lakes. Now these various steams meeting with, and running 

 thiough waters, must cause a great variety of phenomena and 

 effects in them." 



Many or' these depend obviously upon the agency of volcanos, 

 and are immediately connected with them. There are many hot 

 springs, however, whose temperature is too equable, and which 

 occur at too great a distance from any known volcanos to be pro- 

 duced by them. " Thus the hot-spring at Bath," observes Dr. 

 Thomson *, has continued at a temperature higher than that of the 

 air for a period not less than 2uOO years; yet it is so far from any 

 volcano, that we cannot, without a very violent and improbable ex- 

 tension of volcanic tires, ascribe it to their energy. There are va- 

 rious decompositions of mineral bodies, which generate considerable 

 heat. These decompositions are usually brought about by means 

 of water; or, to speak more properly, water is itself the substance 

 which is decomposed, and which generates heat by its decomposi- 

 tion. Thus, for example, there are varieties v of pyrtes, which are 

 converted into sulphate of iron, by the contact of water, and such 

 a change is accompanied by an evolution of heat. Were we to sup- 

 pose the Bath spring to flow through a bed of such pyrites, its heat 

 might be occasioned by such a decomposition. Such, probably, is 



* History of the Royal Society, b. I. ch. iii. 



