I 



226 EAETH'S CKUST A BODY COOLING FEOM FUSION. [Ch. XXXIII. 



tlie several vents which were open on Etna, of no less tlian 

 22,000 cubic metres of aqueous vapour. 



^/ 



fresh water to the volcanic 



/ 



The presence of nitrogen among the gases evolved 



from craters in eruption, and in the waters of thermal 

 springs, has been another subject of enquiry and discussion. 

 Sir H. Davy, in his memoir on the ' Phenomena of Vol- 

 canos,' remarks, that there was every reason to suppose in 

 Yesuyins the existence of a descending current of air j and 



lie imagined that subterranean cavities which threw out 

 large volumes of steam during the eruption, might 



after- 



become 



atmospheric air."^' The presence of ammoniacal salts in 



ammonia 



posed of nitrogen) in lava, favours greatly the notion of 

 air as well as water being deoxidated in the interior of 



the earth. t 



Dr. Daubeny suggests that water containing atmospheric 



air may descend from the surface of the earth to the 

 volcanic foci, and that the same process of combustion by 

 which water is decomposed may deprive such subterranean 

 air of its oxygen. In this manner great quantities of nitro- 

 gen may be evolved. 



The presence of vast numbers of siliceous cases of infu- 

 soria in the tuff covering Pompeii, and composed of matter 

 ejected from Vesuvius and other volcanos, has already been 

 alluded to. (Vol. I. p. 644.) Tho}^ prove that water and 

 mud have penetrated downwards from the surface into rents 

 and caverns in the interior, and have then been thrown out 

 again during volcanic eruptions. 



ling from a state off 



■What 



manner 



aqueous and other 



mechanically 



cally on the crust of the earth, whenever water and various 

 acids, stored up in caverns and fissures at great depths, have 

 their temperatures raised, must satisfy the reader that it is 

 only necessary, in order to explain the action of volcanos, 



^ Phil. Trans. 1828. 



t See Daiibcny, Eiicyc. Metrop. Part 40. 



I. 



I 



I 



> 



J 



4 



< 



i: 



\ 







• 





i 



Ci^ 



s 



<s^ 



to 



(li=' 



-nO 



a 



gucli 

 other, C' 



(Ti-ea 



lakes, 



the? 



at ^a^'^'' 

 their lie 

 others t 



tate, tl 



s 



a mass ( 



but ID ' 



rpo'lOllS 



surface, 



aUv ma] 

 and the 

 That 

 volcano 

 of lava 



phenom 

 been inf 



depths ( 

 have aj 



eciualled 



hold : 

 The 



such 



com 



res. 



n.u- n 



holi 

 ^olca 



1 



^dio 



Un 



n 



e^l 

 t^ 



°ft^u see 



aftof 



e;\h 





Mtl 



1 



th 



e 



