56 The Causes and Phenomena of Earthquakes. 



This latter author assumes that eruptions and earthquakes arise 

 from the access of a column of sea or fresh water, by means of 

 fissures, to the fluid mass below ; and there are many circum- 

 stances which justify this as a sufficient explanation of certain 

 examples. Tyndall has shown, moreover, how force may be 

 represented by heat. Mallet also prefers a deduction from this 

 to the idea of a crust floating on a molten base. 



There is again what is called the Chemical theory, that of Sir 

 H. Davy, which is supported by Dr. Daubeny, but opposed by 

 G-ay Lussac and Bischoff" ; and shown by Hopkins to be mathe- 

 matically impossible. Davy assumed that water and atmospheric 

 air have access to an unoxidised metallic mass, which 

 becomes oxidised, so producing heat. The idea of such a 

 metallic mass was strengthened by the fact that the mean density 

 of the earth, taken in relation to that of water, is 5|, and that 

 many of the rocks have S.Gh from 2 to 3, whilst that of metals 

 ranges up to 21, from which it was concluded that there 

 might be a mean S.Gr. for the central portion of the earth, equal, 

 as assumed by some, to that of iron. Hopkins argues that the 

 molten matter arising in the assumed fissures, would prevent the 

 access of air and water ; and Stromboli and Kirauea are quoted as 

 evidence against it, because the lava has constantly stood fluid in 

 them, at a height considerably above the level of the sea. 



Another objection is stated by Lyell, in his new volume (p. 

 233). It was at first believed by some, including himself, that 

 Davy's theory was insufficient, because no hydrogen was detected 

 by him in Vesuvius. Hydrogen has, however, been detected 

 since in considerable quantities ; and M. "Fouque, in 1865, found 

 that, during the eruption of Etna, no less than 22,000 cubic 

 metres of aqueous vapour were daily emitted. But this was 

 chiefly proto-carbonate of hydrogen, and not free hydrogen, which 

 was concerned, according to Fouque, whereas, if Davy's theory 

 of oxidation holds good, an opposite result ought to be arrived 

 at. Tet both Davy and Fouque agree that water does probably 

 obtain access to the molten fluid, and we have this fact to sustain 

 Bischoff" s theory so far, that live fish have been ejected during 

 an eruption, proving the access of water ; as well as the great 

 amount of infusoria? in the ashes covering the remains of Pompeii, 

 which was destroyed by the ash eruption of Vesuvius in the year 

 79. It is said that great quantities of fish are ejected with 

 torrents of water from a height of from 16,000 to 17,000 feet, 

 during eruptions of Cotopaxi, Tunguragua, and Sangay, in South 

 America. Sir J. Herschel, speaking of sudden explosive action 

 among the extinct volcanos of Auvergne and Vivarais, says that 

 the granite rocks are sometimes perforated without being 

 shattered, as if water had been suddenly let in on a heated 

 molten mass below, producing so violent and sudden 



