554 



DR. DAVY'S REPLY TO DR. DAUBENY. 



Dr. Daubeny has mistaken his meaning, and that the similarity he wished to point out 

 existed not in the bubbling of the gas (that not being peculiar to a chalybeate spring), 

 but in the change of colour of the bottom ; for he says where the air came up most 

 plentifully, there " the cinders (elsewhere quite black) had a rusty appearance.'* 



As to the inferior degree of interest which Dr. Daubeny connects with my manner 

 of explaining the phenomenon, it is of little importance. Truth, of course, or an ap- 

 proximation to it, is the end of inquiry ; and the explanation which I offered had the 

 preference, because it appeared most in accordance with the facts. I would not wish 

 to undervalue Dr. Daubeny's speculations ; but in attaching so much interest to his 

 opinion, that the atmosphere is the source of the gas disengaged, derived through 

 subterranean channels, I cannot help thinking he has had recourse to a difficult 

 hypothesis of little usefulness ; for what facts are there in favour of the idea that 

 volcanic fires are fed like ordinary fires, or in any way dependent on the atmosphere 

 for their activity ? 



That azote is often a product, and an abundant one, of extinct volcanos is certain ; 

 but it does not follow that it is also a product of active volcanos. Probably the am- 

 moniacal salts which form in such abundance in certain solfataras, of which that of 

 the island of Volcano is the most remarkable, is owing to a complicated play of 

 chemical affinities, in which atmospheric air, sulphur, alumine, and steam are the ele- 

 ments chiefly concerned. 



Dr. Daubeny concludes his note by expressing the wish that the quantity of gas 

 evolved by thermal springs should be ascertained. This is a scientific desideratum ; 

 but some caution is required how the knowledge so obtained is made a test of the 

 truth of any theory of the origin of the air in such springs. Atmospheric air may be 

 carried down not only dissolved in water, as in the rains feeding springs, as I have 

 dwelt on in my former paper, but it may be also forced down mechanically in froth 

 by the impetus of a descending stream of water, and, so entangled, may find its way 

 to a great distance ; and according to the nature of the strata and channels through 

 which it passes, it may either lose oxygen by the attraction of metallic bodies, or 

 have its oxygen converted into carbonic acid by the action of carbonaceous matter, 

 or it may ascend unaltered. A remarkable instance of this last-mentioned condition 

 presents itself in the springs of the Sava, about a quarter of a mile above Wurzen. It 

 is recorded in my brother's Journal, in which, on the 27th of August 1828, he has 

 written : " Admired the sorgente Sava, — a number of deep circular holes, with air 

 bubbling through them, and large jets of water, which is beautifully clear." And on 

 the 30th he added : " Examined this evening the air disengaged in such large quantities 

 where the Sava rises. It appeared to me to possess all the characters of common air, 

 was not absorbable by water, and supported flame in the same manner as common air." 



Malta April 20, 1834. 



