Site of the Ancient City of the Auranci. 253 



caused by a disengagement of carburetted hydrogen, derived from 

 the bitumen which Knox discovered in certain obsidians ; but this 

 cannot be the case, as the presence of bitumen is an exception rather 

 than the rule. It is more probable, that the formation of pumice is 

 connected with the disengagement of chlorine, derived from sea-salt, 

 which maybe decomposed by the heat, its soda being seized upon by the 

 mineral, and entering, as before explained, into its composition. — A 

 portion of this chlorhio, however, adheres most tenaciously to the mi- 

 neral mass, which is proved by the fusion of pumice into glass, as even 

 then it retains a portion of this volatile ingredient. With respect to 

 obsidian, Abich conceives, that the greater and more continued the 

 pressure to which the melted material has been subjected may be, 

 the greater tendency will be shewn by the mass, both to assume a 

 stony rather than a vitreous texture, and to form definite and distinct 

 minerals, rather than one homogeneous amorphous compound. Hence 

 from the same material lithoide lavas may be produced under pres- 

 sure, vitreous ones in the open air. 



Appendix III. (Page 247.) 

 On the Chemical Theory of Volcanoes. 



Professor Bischof justly observes, ** that the close connection be- 

 tween volcanoes and hot springs would lead us to refer both to the 

 same cause." " But," he continues, " thermal springs are too uni- 

 versally distributed to be accounted for by chemical processes going 

 on in the interior of the globe. They seem to occur everywhere where 

 the water rises from a great depth. They must, therefore, be attri- 

 buted to the high temperature which generally pervades the interior 

 of the globe." 



To this I would reply, that no one questions that the high tempe- 

 rature of a spring is acquired immediately from that of the rock from 

 whence it proceeds, or supposes the rock, when it lies at a distance 

 from any volcano, to derive its heat from chemical processes taking 

 place within itself. From a mineral mass so circumstanced, as well 

 as from a water simply thermal, w^e can collect nothing which should 

 lead us to give the preference to either theory ; it is only from ana- 

 logy, that the heat either of the one or of the other can be explained. 

 But more commonly, hot springs, like volcanic eruptions, are accom- 

 panied with other products, which seem to imply the existence of 

 chemical action ; hot springs, for instance, with carbonic acid, sulphu- 

 retted-hydrogen, and azote, — ejections of lava, with steam, muriatic 

 acid, sulphuretted-hydrogen, and ammonia. The carbonic acid, indeed, 

 might be evolved from limestone, owing to the mere access to it of heat ; 

 but I cannot agree with Professor Bischof in attributing the sulphu- 

 retted-hydrogen to a decomposition of sulphates by organic matter. 

 Sulphuric salts do not occur in the majority of these springs, and the 

 small quantity therein existing of baregine (the only organic matter 

 which is present) appears to be generated after the water has reached 



