CATARACTS, AND INUNDATIONS. l6l 



sometimes found of 'the usual temperature, and not unfrequently 

 raised far beyond it, of which we have already given a few exam- 

 pies in one or two of the preceding sections. The heat, as we have 

 also already hinted, appears sometimes from active volcanos in 

 the immediate vicinity ; and sometimes from the disengagement of 

 subterranean gasses powerfully combined with caloric. In many 

 instances the heat appears to be generated by the decomposition of 

 water itself. Thus, for example, there are varieties of pyrites 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 suppose the Bath spring to flow through a bed of such pyrites, 

 its heat might be occasioned by a decomposition of this kind. Such, 

 probably, is the way in which those mineral springs, that con- 

 tain sulphureted hydrogen gas, receive their impregnation. But 

 we are pretty certain, that such a supposition will not apply to Bath 

 water : first, because it does not contain the quantity of sulphate 

 of iron, which would be necessary upon such a supposition ; and 

 secondly, because instead of sulphureted hydrogen gas, which 

 would infallibly result from such a decomposition of pyrites, there 

 is an evolution of azotic gas. This evolution of azotic gas, however, 

 is a decisive proof that the heat of Bath waters is owing to some 

 decomposition or other, which takes place within the surface of the 

 earth ; though, from our imperfect acquaintance with the nature of 

 the mineral strata, through which the water flows, we cannot give 

 any satisfactory information as to what that decomposition actu- 

 ally is. 



In treating this important subject in as popular a way as a some* 

 what exact and scientific enquiry will allow us, we shall first take a 

 brief survey of the principal medicinal waters of foreign countries ; 

 next of those that are domestic or belong to our native soil ; 

 and then examine the general nature and proportion of the mi- 

 neral substances that enter into them, 'and point out the means of 

 detecting and analysing them. 



EDITOR. 







2. Principal Foreign Medicinal Waters* 



. 



IN the peninsula of Kampschatka are several hot springs, pos- 

 sessing very singular properties. Captain King speaks of one at 



VOL, III. M 



