and their Connexion with Volcanos. 76 



themselves, are left unexplained. Still less does the hypothesis 

 pretend to account for the emission of aqueous vapour, or of the 

 various aeriform fluids which are so commonly present. 



Considerably higher in the scale of probability, is that modifi- 

 cation of the above theory advocated by my friend Professor 

 Lyell *, in which the phenomena are deduced from the occa- 

 sional descent of a body of water derived from the sea into the 

 interior of the earth, where it meets with a mass of matter in an 

 incandescent, if not in a fluid, condition. 



We have here an explanation of more at least of the circum- 

 stances of the case, such as the emission of steam, the occur- 

 rence of sea-salt, and the general position of volcanos near the 

 coast ; and we have likewise an ingenious cause assigned -|* for 

 the intermittent character of the eruptions, and for the force 

 with which the lava is propelled upwards to the spots at which 

 it finds a vent. But the hypothesis does not sufficiently ex. 

 plain the nature of the chemical products evolved, and more 

 especially that of the gases which accompany its operations in 

 all their various stages. 



Even the chemical theory, as originally propounded, leaves 

 out of sight the constant production of nitrogen, or of ammoni- 

 acal salts derived from it, in all volcanic processes, and cannot 

 be pronounced adequate to account for the phenomena, unless 

 it be modified by the admission, that atmospheric air as well as 

 water is concerned in maintaining that combustion which is in- 

 dicated as its cause. 



The circumstance, however, which has most contributed to 

 give a currency to the mechanical hypothesis, is the general be- 

 lief entertained of the high temperature existing in the interior 

 of the globe, which, as it appears to imply that there must be 

 occasional vents for the subterranean fire hence supposed to exist, 

 has induced a reluctance in the minds of many to calling in the 

 aid of any new and hypothetical principle. 



Undoubted! 'the idea of a central heat has to boast the autho- 

 rity of many distinguished supporters, though it must be re- 

 marked at the same time, that one of the profoundest of the phi- 

 losophers who have entertained this opinion, admits, that it de- 

 rives no sort of confirmation from any phenomena observable on 



• See Lyell's Geology, p. 466. et seq. f lb. p. 460. 



