82  _—s- Reply of Dr. Daubeny to Prof. Bischof. 
sessing a higher enh gravity than that of the compound resulting from the oxi- 
dation of the entire mass. 
Let us take for SERIA the recs baa given by Dr. Kennedy, of the lava from 
ag which he states to consist 
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 Tron, 15 per cent X Sp. gr. 5.00 = 75.0 
Soda, 4 per cent 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 32, 
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 
Cheeposiu on. 
Silica, 52, contains of base, 26 % Sp. gr. 2.0 = 520 
Alumina, 19, contains of base, 10 % Sp. gr 2.0 = 20.0 
Lime, 10, contains of base, 7 yx Sp. gr. 4.0 = 28.0 
Oxide of Iron, 15, contains of base, 12 X Sp. gr 7.8 = 93.6 
oda, 4, contains of base, 3 X Sp. gr. 10= 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. 
specific gravity of aluminium appears not to be ascertained, but probably it is not 
inferior to that of —— which sinks in the strongest sulphurie acid, and there- 
fore is more than 
The theory, thicohrs, we have been advocating, leaves the question, with re- 
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 t 
implies merely the existence of such a quantity of metallic ingredients, as would 
be sufficient to peeins the materials hip leaving the constitution of the re 
mainder just as open to conjecture as it was before, 
It is curious Sedenk. that whilst some pet 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 ¢om 
tended, that these substances would have their specific gravity so much increased 
by the enormous pressure from above, that void internal spaces must be nece* 
sarily su pposed. 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 Kechase it appears to militate against the 
conjectures that may be conjured up with regard to the internal conditio n of our 
planet.” 
