82 Notice of Active and Extinct Volcanos. 



process are compressed by the superincumbent mass of rock, un- 

 til they enter probably into new combinations, or diffuse them- 

 selves through the solid strata. 



" But under the sea, where the pressure of an enormous co- 

 lumn of water assists in forcing that fluid through the minutest 

 crevices in the rock, the action must go on more rapidly, and the 

 effects consequently be of a more striking nature. 



" These effects however will take place in the middle of the 

 sea less generally than on the coast, because the pressure of the 

 ocean itself opposes an impediment; and it will in general not be 

 constant, but intermittent, because the heat generated by the pro- 

 cess itself will have a tendency to close the aperture by which 

 the water entered, first, by injecting the fluid lava into the fis- 

 sure, and secondly, by causing a general expansion of the rock; 

 nor will the water again find admission, until, owing to the cessa- 

 tion of the process, the rock becomes cool, and consequently again 

 contracts to its original dimensions. 



" Now the first effect of the action of water upon the alkaline 

 and earthy metalloids will be the production of a large volume of 

 hydrogen gas, which, if air be present, will combine with oxygen 

 and return to the state of water, if it be absent, will probably com- 

 bine with the sulphur, both being at the high temperature favor- 

 able to their union. In the former case nitrogen gas will be giv- 

 en off, in the latter sulphuretted hydrogen. 



" But in case of the presence of oxygen, the sulphur will also 

 become inflamed, and give rise to the production of sulphurous 

 acid, which will predominate among the gaseous exhalations emit- 

 ted from the mouth of the volcano, provided sufficient quantity of 

 air be present to combine with the hydrogen and re-convert it 

 into water. So soon however as the oxygen is consumed, the hy- 

 drogen, no longer entering into combustion, unites with the heat- 

 ed sulphur, and escapes in the form of sulphuretted hydrogen, 

 which, towards the latter period of the eruption, will predominate 

 over the sulphurous acid, because it continues to be formed long 

 after the want of oxygen has put a stop to the production of sul- 

 phurous acid. Now it is well known, that these two gases mutu- 

 ally decompose each other, and therefore cannot exist at the same 

 time, so that the appearance of sulphuretted hydrogen from the 

 mouth of the volcano may indicate, if not the entire absence of sul- 

 phurous acid at the place at which the process takes place, at 

 least that its formation is stopped by the consumption of oxygen, 

 or is going on with less energy than heretofore. 



"The very circumstance of the reproduction of water by the 

 mutual decomposition of these two gases, might be the means of 

 keeping up the action in a languid manner for an indefinite peri- 

 od. The slowness with which lava cools would cause it to give 



