Chemical Theory of Volcanos, 297 



Now, let us contrast this -with the specific gravity of 100 parts of the 

 metallic principles, which would give rise to a mineral possessing the 

 above chemical composition. 



Silica, 62, contains of base, 26 x sp. gr. 2.0 = 62.0 



Alumina, 19, contains of base, 10 X sp. gr. 2.0 = 20.0 



Lime, 10, contains of base, 7 X sp. gr, 4.0 = 28.0 



Oxide of Iron, 15, contains of base, 12 X sp. gr. 7.8 = 93.6 

 Soda, 4, contains of base, 3 X sp. gr. 1.0 = 3.0 



100 68 196.6 



Now as 58—196—100—340. 

 Consequently the specific gravity of the whole would be no less than 

 3.4. The specific gravity of aluminium appears not to be ascertained, 

 but probably it is not inferior to that of silicon, which sinks in the strong- 

 est sulphuric acid, and therefore is more than 1.83. 



The theory, therefore, we have been advocating, leaves the question, with 

 respect to thecause of the Earth's density, just on the samefooting as before. 

 Those who are of opinion, that the latter may be explained by the mere 

 condensation of such rocks as are found near the surface, in consequence 

 of the superincumbent weight, as certain metals may be rendered heavier 

 by pressure, are entitled to extend this explanation to the case of the al- 

 kaline and earthy bases ; whilst those who regard the density of the 

 Earth to be a proof that some heavier matter must exist below, are not 

 precluded from such a supposition, as our theory implies merely the ex- 

 istence of such a quantity of metallic ingredients, as would be sufficient 

 to produce the materials ejected, leaving the constitution of the remain- 

 der just as open to conjecture as it was before. 



It is curious indeed, that rvhilst some have argued, that the kind of 

 materials found near the surf, -e is inadequate to account for the density 

 attributed to the Earth in general ; others, as the late distinguished Pro- 

 fessor Leslie, have contended, that these substances would have their 

 specific gravitj'^ so much increased by the enormous pressure from above, 

 that void internal spaces must be necessarily supposed. On this he has 

 founded his singular h}^othesis, that the centre of the Earth is filled only 

 with light, the rarest substance known j an idea, the mere mention of 

 which is sufiicient to shew how little we can be justified in rejecting an 

 explanation of facts, merely because it appears to militate against the 

 conjectures that may be conjured up with regard to the internal condi- 

 tion of our planet." 



Sth Objection, If, according to Gay-Lussac, the hydrogen of 

 the decomposed water goes to form muriatic acid with chlorine, 

 the above mentioned acid ou<jht to be general in volcanos. 

 Now, it is wanting, according to Boussingault, in the volcanos 



VOL. XXVI. NO. Lll. APRIL 1839. U 



