( 158 ) 



Note by Dr Daubeny to his Memoir, contained in the Edin- 

 burgh New Philosophical Journal for April 1839, in Beply 

 to Professor Bischof^s Bemarks on the Theory of Volcanos. 

 (Communicated by tbe Author.) 



I hasten to correct an error, which a friend has had the 

 goodness to point out to me, in the calculation entered into 

 in my reply to Professor Bischof, concerning the relative 

 weight of the materials of which ordinary lava consists, com- 

 pared to that of the metallic bases which are included in its 

 composition. 



In this calculation I had erroneously estimated the specific 

 gravity, both of the lava, and of its metallic constituents, 

 from the proportion of their respective ingredients by weight, 

 and not, as ought to have been done, from that of their re- 

 spective hulks ; thus giving to the lava itself a specific gravity 

 of 3.206, and to its metallic constituents one of 3.40 ; whereas 

 the true specific gravity of the former turns out to be 3.09, 

 and that of the latter only 2.39. 



Although I felt the propriety of correcting this inadver- 

 tence from the moment it had been pointed out to me by my 

 intelligent correspondent, I do not conceive that my argument 

 is materially weakened by its admission. 



Whilst the chemical theory of volcanos supposes those por- 

 tions of the interior of the globe in which volcanic operations 

 take place, to contain, among other constituents, the metallic 

 bases of the substances which are found to constitute lava, 

 the opposite hypothesis necessarily assumes, that the mate- 

 rials of the lava themselves existed before their ejection in the 

 same position. 



But, on either supposition, we have equally to encounter the 

 difficulty arising from the high specific gravity which mathe- 

 maticians have assigned to the internal contents of our globe, 

 and consequently are driven to the alternative of imagining, 

 either that some denser material exists at a still greater depth 

 than that from which the lava issues, or that the lava itself 

 has had its density proportionally increased by the superin- 

 cumbent pressure. 



Now, under such circumstances, the difl&culty would be not 



