294 Dr Daiibeny oji the 



between these caverns and the external atmosphere were her- 

 metically sealed (which no one contends), air must at times en- 

 ter the latter to fill the vacuum thus occasioned. ♦ 



2d Objection. — If the oxidation of the earthy and alkaline 

 metals were to take place at the expense of water, enormous 

 quantities of hydrogen would be evolved, which has never 

 been observed. 



Answer. — Hydrogen could hardly be expected to escape in 

 a free state from a spot which contained so many elements for 

 which it possesses a strong affinity, and to which it would be 

 presented under the influence of the pressure and temperature 

 so well calculated to promote its combination with them. 



Thus, sulphur and chlorine we know to be generally pre- 

 sent in volcanos, and oxygen and nitrogen, we may fairly as- 

 sume to be so. But, although hydrogen may not be disengaged 

 alone, large quantities of it in combination with sulphur appear 

 to be almost universally evolved from volcanos, and it is proba- 

 ble that the great beds of sulphur which exist in most volcanic 

 districts (viz. Sicily) are the result of the decomposition of the 

 sulphuretted hydrogen evolved. Nor, indeed, does it seem pos- 

 sible to explain the presence of this hydrogen, without having 

 recourse to the chemical theory. 



4^/i Objection. — The evolution of carbonic acid by volcanos 

 is not explained, and these disengagements of carbonic acid 

 gas could not take place in the presence of atmospheric air in 

 those vast subterranean cavities without their mixing together. 

 Now, the carbonic acid evolved by volcanos (Vesuvius, Eifel, 

 &c.), contains but little atmospheric air. 



Answer. — The evolution of carbonic acid in countries ex- 

 posed to the influence of volcanic heat, would seem to be a ne- 

 cessary result of the existence of calcareous matter in the rock 

 formations. Its continuance for so long a period after the vol- 

 cano has ceased to be in activity, seems to shew, that it is not de- 

 rived directly from the chemical processes which produce the 

 phenomena in question, but is only caused by the heat which 

 these processes tend to diffuse through the adjacent rocks. Hence 

 there seems no reason why it should be intermixed with any 

 large proportion of common air, though, as I have shewn, this 



