82 Reply of Dr. Dauheny to Prof. Bischof. 



sessing a higher specific gravity than that of the compound resulting from the oxi- 

 dation of the entire mass. 



Let us take for instance, the analysis given by Dr. Kennedy, of the lava from 

 Etna, which he states to consist of 



Silica, 52 per cent X Sp. gr. 2.65 = 127.8 



Alumina, 19 per cent X Sp- gr- 4.20 = 79.8 



Lime, 10 per cent X Sp. gr. 3.00 = 30.0 



Oxide of Iron, 15 per cent X Sp. gr. 5.00 = 75.0 



Soda, 4 per cent X Sp. gr. 2.00 = 8.0 



100 320.6 



We here find that 100 parts of this lava have a specific gravity equal to 320.6, 

 and consequently that the specific gravity of the mass would be no more than 3.2, 

 supposing it divested of water. 



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, 52, contains of base, 26 X Sp. gr. 2.0 = 52 



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 ^^ 936 



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



100 58 196.6 



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 strongest sulphuric acid, and there- 

 fore is more than 1.83. 



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

 spect to the cause of the Earth's density, just on the same footing as before. 

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

 sation of such rocks as are found near the surface, in consequence of the super- 

 incumbent weight, as certain metals may be rendered heavier by pressure, are 

 entitled to extend this explanation to the case of the alkaline 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 existence of such a quantity of metallic ingredients, as would 

 be sufficient to produce the materials ejected, leaving the constitution of the re- 

 mainder just as open to conjecture as it was before. 



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

 found near the surface is inadequate to account for the density attributed to the 

 Earth in general; others, as the late distinguished Professor Leslie, have con- 

 tended, that these substances would have their specific gravity so much increased 

 by the enormous pressure from above, that void internal spaces must be neces- 

 sarily supposed. On this he has founded his singular hypothesis, that the centre 

 of the Earth is filled only with light, the rarest substance known ; an idea, the 

 mere mention of which is sufficient to show how little we can be justified in re- 

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

 conjectures that may be conjured up with regard to the internal condition of our 

 planet." 



