On the Agency of Water in Volcanic Eruptions. 



125 



elements, and in so dissociating it augments its volume by one half, 

 and its pressure in proportion.* That water is decomposed in contact 

 with lava during eruptions, is rendered probable by the observations 

 of M. Fouque daring the last great eruption of Santorin, for he 

 found that the gases given off under water during the eruption, and 

 collected as they ascended through the sea, often contained as much 

 as 30 per cent, of free hydrogen, and from the circumstance that he 

 also found free oxygen occasionally present, he considered it likely 

 that the vapour of water exists iu a state of dissociation in the lava 

 during eruptions. f 



Other geologists have contended for the possibility of water gaining 

 access to the volcanic foci by fissures opening into the sea-bed. % 

 These fissures are supposed to be formed by the molten matter 

 struggling to escape. To this it has been rightly objected, that in 

 such a case the lava would at once fill the fissure to the exclusion of 

 the water. By others it lias been suggested that the fissures are 

 caused by the escape of imprisoned elastic vapours ; but as Mr. Scrope 

 remarks, this is reasoning in a circle, for while it supposes the 

 aqueous vapour to be the cause of the disturbance, it yet proposes to 

 introduce the water after the effects attributed to it had been pro- 

 duced. 



The second objection is, that supposing it were possible for water 

 to penetrate to the molten magma and to be converted into high 

 pressure steam, would it be possible for it to force forward and 

 gradually erupt a column of lava extending from the molten mass 

 below to the volcanic summit ? Bischof's§ hypothesis was founded on 

 an erroneous estimate of the elastic force of steam. 



Would not also, on the fissure hypothesis, the pent up elastic 

 vapours, which are supposed to force the lava up the volcanic duct, 

 necessarily take the line of least resistance, drive back the column of 

 water in the fissure and escape with it ? 



But the objection to which I attach most weight and importance is 

 one which deals with facts which are within the scope of actual 

 observation. On the hypothesis that attributes the extrusion of lava 

 to " the expansive force of some elastic aeriform fluid, struggling to 

 escape from the interior of a subterranean body of lava," it would 

 follow that no lava could escape without the accompaniment of the 

 propelling aeriform fluid, nor could any large evolution of vapour 

 or gases take place without a large eruption of lava, for the relative 



* At the same time it is to be observed, that enclosed in a platinum tube water 

 does not decompose at a temperature near the fusing point of the platinum, 

 f " Santorin et ses Eruptions," p. 232. 



X " Bull. Greol. Soc. de France," vol. xiii, p. 178; vol. xvi, p. 43; and 2nd Ser., 

 vol. i, p. 23. 



§ " Edin. New Phil. Jour.," vol. xxvi (1839), p. 132. 



