N 



224 ACCESS OF SALT WATER TO VOLCANIC FOCL [Ch. XXXIIL 



t 



prevalent where there is an incumbent body of salt water, or 

 in any regions rather than in the interior of a continent, where 

 the quantity of rain-water is reduced to a minimum. The 

 experiments and observations of the most eminent chemists 

 have gradually removed, one after another, the objections 

 which were first offered to the doctrine that the salt water 

 of the sea plays a leading part in most volcanic eruptions. 

 Sir H. Davy observed that the fumes which escaped from the 

 Vesuvian lava deposited common salt.* 



M. Gay-Lussac, although he avowed his opinion that the 

 decomposition of water contributed largely to volcanic action, 

 called attention, nevertheless, to the supposed fact, that 

 hydrogen had not been detected in a separate form among 

 the gaseous products of volcanos ; nor could it, he said, be 

 present ; for, in that case, it would be seen inflamed in the 

 air by the red-hot stones thrown out during an eruption.f 



But M. Abich remarked, on the other hand, ' that although 

 it be true that vapour illuminated by incandescent lava 

 has often been mistaken for flame,' yet he had clearly 

 detected the flame of hydrogen in the eruption of Vesuvius 

 in 1834.t 



In the memoir just alluded to, M. Gay-Lussac expressed 

 doubt as to the presence of sulphurous acid ; but the abun- 

 dant disengagement of this gas during eruptions has been 

 since ascertained: and thus all difficulty in regard to the 

 general absence of hydrogen in an inflammable state is 

 removed ; for, as Dr. Daubeny suggests, the hydrogen of 

 decomposed water may unite with sulphur to form sulphu- 

 retted hydrogen gas, and this gas will then be mingled with 

 the sulphurous acid as it rises to the crater. It is shown 

 by experiment, that these gases mutually decompose each 

 other when mixed where steam is present ; the hydrogen of 



the one immediately uniting with the oxygen of the other 



to form water, while the excess of sulphurous acid alone 

 escapes into the atmosphere. Sulphur is at the same time 



precipitated. 



Davy, Phil. Trans. 1828, p. 244. 

 t Ann. de Chim. et de Pliys. torn, xxii 

 Phenom. Geol. &c. p. 3. 



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