Volcanos and Earthquakes, 31 



lius* haB shewn, that silicium, the combustible base of silica, 

 when freed of hydrogen by being gradually heated to a white 

 heat, is incombustible even at that heat in the air or in oxygen ; 

 and that it is equally incapable of decomposing water. In like 

 manner Wiihlerf found, that aluminum, the metallic base of 

 alumina, is not oxidized under a red heat, and decomposes hot 

 water but very slowly, while on cold water it has no influence 

 whatever. 



Therefore Davy'^s hypothesis would be applicable only to the 

 metallic bases of alkaline earths and alkalies. But, as these oc- 

 cur only in small proportions in the volcanic rocks, it is scarce- 

 ly conceivable that so much heat should be evolved by their 

 combustion at the ordinary temperature as would be sufficient 

 to melt the pure earths, or to inflame their metals, supposing 

 them to exist at the seat of the volcanic action. 



The shght specific gravity of the metals of the alkalies, also 

 proves fatal to Davy^s hypothesis ; for, i£ the mean density of 

 the earth surpass that of all kinds of rocks, those metals can- 

 not exist, at least not in great quantities, in the interior of the 

 earth.J Davy's hypothesis, therefore, according to the present 

 state of science, will not account for volcanic phenomena. § 



Gay-Lussac,|| assuming that water supplied the oxygen in 

 volcanos, endeavoured to account for the absence of uncombined 

 hydrogen among the exhalations of volcanos, by supposing it 

 to form such combinations with other bodies as would not in- 

 flame by coming into contact with the air. This is the case 

 when it combines with chlorine to form muriatic acid gas. He 

 here refers to the observations of Breislak,^ and of Monti- 



* Poggend. Ann. v. i. p. 221. f Ibid. t. xi. p. 146. 



X Also the latest experiments, made with admirable exactness by Pro- 

 fessor Reich in Freiberg^ with tlie assistance of the torsion-balance, liave 

 given 5.44 for the density of the earth, as a mean of 14 experiments which 

 afforded very nearly the same results. Yersiiche Uber die mittlere Dichtig- 

 keit der Erde mittelst der Drehwage von F. Reich. Freiberg 1838. This 

 j-esnlt accords very nearly with that, which was found by Cavendish and 

 Hutton. 



§ Davy, however,, afterwards abandoned his hypothesis. See Consola- 

 tion ia Travel, or the Last Days of a Philosopher. 



II Loco cit. ' IF Loco cit. iii. p. 57 and 94. 



