G. Poulett Scrope — The Mechanism of Stromboli. 537 



The mechanism by which the rise of lava and steam within the 

 vent is effected was explained by myself so early as 1825, in the 

 first edition of my work on Volcanos (pp. il7-20), and more fully 

 still in the edition of 1862 (pp. 39-43). It corresponds precisely 

 with the view talven by Lyell in the " Principles " (tenth edition, 

 vol. ii. p. 225), and also with that given by Dana (Manual, 1863, 

 p. 692) in the following passage, which I quote in preference to 

 my own, as the view of an impartial and unquestionable authority : 



" Volcanic Phenomena. — The water and other vaporizable sub- 

 stances within the lava" (in the chimney of a volcano) "are under a 

 pressure of about 125 pounds to the square inch for every 100 feet 

 of depth. Owing to the heat, and their consequent expansion, they 

 slowly rise in the heavy viscid liquid. As they rise, they keep ex- 

 panding, until, nearing the surface, they begin to take the form 

 of bubbles of vapour, and finally break through. . . If the lavas 

 are less liquid, the vapours are kept from escaping by the resis- 

 tance, until they have collected in far larger bubbles ; and when such 

 bubbles burst, the projectile force may be enormous; it carries the 

 fragments far aloft, whence they descend in a shower of cinders 

 of great extent. . . Such bubbles, rising and bursting, were seen 

 by Spallanzani in the crater of Stromboli. In times of moderate 

 action at Vesuvius, the outbursts of cinders occur every three to ten 

 minutes ; but in a period of eruption, they are almost incessant. 

 . . As the vaporizable substances (water, sulphur, etc.) and 

 atmospheric air expand while rising in the volcanic vent, they dis- 

 place correspondingly the lava, and so cause a general expansion of 

 the mass. This alone produces a rise of the lavas in the conduit." 



It is then certain that, according to all trustworthy authorities, the 

 outbursts of steam and other vapours, which during volcanic erup- 

 tions rise with and carry up to great heights in the air lava drops and 

 fragments of rock, are seen in all cases to proceed from the lava itself, 

 and not from any separate source of hot water, such as Mr. Mallet's 

 ingenuity has invented. This has been recognized by every observer, 

 who has had the good fortune to witness a volcanic vent in eruption 

 from a commanding post, from the time of Spallanzani and Dolomieu 

 to the present day. 



Mr. Mallet, however, entertains very different ideas upon this sub- 

 ject. It seems that he is unwilling to admit that the water which, 

 as steam, plays so important a part in A'olcanic eruptions, is contained 

 within the lava as it rises in the vent from the interior of the volcano, 

 and forces its way out explosively from the exposed surface on 

 reaching the open air. Hence his determination to find or suggest 

 some other sources of water and steam outside of the lava. Hence, 

 too, he is unable to conceive the possibility of successive volumes of 

 vapour struggling upwards within a narrow and intricate volcanic 

 chimney, filled with more or less viscid lava, and successively burst- 

 ing from its upper surface as they reach the atmosphere in the 

 manner described by Dana in the above-quoted passage. Mr. 

 Mallet asserts (E.S. Proceedings, p. 510) : " Whatever be the source 

 of supply of the lava, it can never fill the tube as a solid {sic) 



