296 Dr Daubeny on the 



cess, generate heat sufficient to cause the other bases to enter 

 into combination with oxygen. Besides, we know that alumi- 

 nium and magnesium enter readily, with an evolution of heat 

 and light, into combination with chlorine, a body which (as 

 we shall see) there is good reason for supposing present in 

 volcanos. 



^ith Objection. — The slight specific gravity of the metals of 

 the alkalies proves fatal to Davy's hypothesis, for, if the mean 

 density of the earth surpass that of all kinds of rocks, these 

 metals cannot exist, at least not in great quantities, in the in- 

 terior of the earth. 



In reply to this I cannot do better than extract the remarks 

 which I made in reply to the self same objection in my article 

 on volcanos, published in the Encyclopaedia Metropolitana in 

 the year 1833. 



" An objection against our hypothesis, has also been sometimes de- 

 duced from the mean density of the Earth, which is calculated at five 

 times that of water ; and hence it has been concluded, that bodies so 

 light as potassium and sodium are, cannot make a part of its nucleus. 



But we are not obliged to imagine a larger proportion of these alka- 

 line bases to be present, than would be implied by the composition of the 

 lava emitted, and probably we shall find not more than four or five per 

 cent, of potass or soda to exist, in the average of volcanic productions. 



On the other hand, the specific gravity of the basis of silica, and pro- 

 bably, also, of that of the other earths which predominate in lava, is SFuf- 

 ficiently considerable to warrant the conclusion, that a mass of matter, 

 containing these principles in the proportions indicated, and united With 

 as much metallic iron as we know to exist in the state of an oxide in 

 the generality of lavas, would form an aggregate possessing a higher spe- 

 cific gravity, than that of the compound resulting from the oxidation 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. 2.66 = 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, 16 per cent X Sp. gr. 6.00 =: 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. 



