Sir H. Davy on the Phenomena of Volcanoes. 85 



great abundance. Treated by the blowpipe with nitrate of 

 cobalt, it assumes the fine blue colour which denotes alumina. 



It dissolves almost entirely in caustic potash ; and this so- 

 lution affords by nitric acid a precipitate which is redissolved 

 by an excess of acid. This last solution is precipitated by 

 ammonia and by barytes. 



These experiments sufficiently prove the presence of water, 

 alumina, and sulphuric acid, and also the absence of silica. 



The complete analysis by M. J. Dumas gives 



Sulphuric acid 23 



Alumina 30 



Water 47 



and this result agrees in the constituent principles and their 

 proportions with those of the Websterite of Halle and New- 

 haven. 



Thus, we perceive, as I have stated in the commencement 

 of this notice, that this friable substance, which has more the 

 appearance of an adventitious earthy mixture than of a mineral 

 species, presents in its composition, an identity of principles, 

 together with a precision in their proportions, rarely found in 

 crystallized minerals, which indicate, by their solidity and lim- 

 pidity, species completely limited. We see it also placed in 

 geological positions and circumstances of which the constancy 

 is no less striking. There is, however, between the Web- 

 sterite of Autueil, and that of other localities, a slight differ- 

 ence of structure, which may serve to establish a variety in this 

 species. It has the oolitic structure ; and we may therefore 

 distinguish it by the name of Oolitic Websterite of Auteuil. 



XIII. On the Phenomena of Volcanoes. By Sir Humphry 

 Davy, Bart. F.R.S.* 



WHEN in the years 1807 and 1808 I discovered that the 

 alkalies and the earths were composed of inflammable 

 matter united to oxygen, a number of inquiries suggested 

 themselves with respect to various parts of chemical science, 

 some of which were capable of being immediately assisted by 

 experiment, and others required for their solution a long series 

 of observations, and circumstances obtained only with diffi- 

 culty. Of the last kind were the inferences concerning the 

 geological appearances connected with these discoveries. 



The metals of the alkalies, and those of such of the earths 

 as I had decomposed, were found to be highly combustible, 

 and altered by air and water even at the usual temperatures 



* From the Philosophical Transactions, for 1828. Part I. 



of 



