282 Remarks on Volcanoes. 



agency of the hydrates of the alkalies or alkaline earths, 

 assisted hy a high temperature, during which, as he has 

 shewn, hydrogen is evolved ; and he even shews that if sul- 

 phur in vapour be brought into contact with basalt at a high 

 temperature, and afterwards steam be passed over the rock 

 so treated, sulphurous acid is disengaged in the first instance 

 by the union of the sulphur with the oxygen of the peroxide 

 of iron, which metal forms, with another portion of the same 

 body, sulphuret of iron ; and that sulphuretted hydrogen will 

 be emitted in the second instance, owing to the decomposi- 

 tion of water, and the union of its hydrogen with the sulphur 

 of the pyrites, whilst its oxygen forms, with the metallic por- 

 tion, magnetic oxide of iron. 



Supposing the formation of palagonite to be going on at 

 all times when sulphuretted hydrogen and pure hydrogen can 

 be shewn to be concomitants of the volcanic action, and on a 

 scale commensurate to the amount of gas generated, the ex- 

 planation of Professor Bunsen will probably be accepted by 

 chemists in general, in preference to that which refers it to 

 the decomposition of water by alkaline and earthy metalloids, 

 or their yet unoxidized sulphurets ; but I cannot admit, as a 

 valid objection to this latter hypothesis, the absence of car- 

 bonic oxide from volcanic exhalations, of which carbonic acid 

 constitutes so large a proportion. No doubt the latter would, 

 as Bunsen remarks, be partially converted into carbonic 

 oxide by hydrogen at the high temperature which probably 

 exists around the focus of the volcanic action; but I have 

 always been accustomed to refer the carbonic acid given off 

 by volcanoes to the diffusion of heat over contiguous lime- 

 stone rocks, and not to processes going on at the point where 

 the temperature was most intense. 



Nor do 1 feel quite satisfied with the explanation offered 

 by the Professor, of the presence of sal-ammoniac in the lava, 

 which he refers to the vegetable matter existing in meadow- 

 land overflowed by the molten current. If such were the 

 origin of the volatile alkali, we ought not to find it exhaled 

 round the orifices of the crater, or from any of the fumaroles 

 proceeding directly from the same internal focus of action. 



It is not my purpose, however, especially on such an occa- 



