Lithion-Mkajrom Zinnxoald. 223 



even if this loss is taken for pure water without fluoric acid, 

 there still remains a loss of 3.4 per cent., which, in fact, is 

 very considerable. I have been convinced, by direct ex- 

 periment, that when this mica is fused in a platina crucible, 

 by far the greatest part, if not allthefluoric acid, remains behind ; 

 for, when the fused mass is covered by carbonate of soda, and 

 again exposed to heat, there is obtained by analysis a quantity 

 of fluoric acid, which is probably not inferior to that obtained 

 from the mica before it has been previously fused. The con- 

 siderable loss may be, I think, accounted for by the volatiliza- 

 tion of the alkaline matter, which is expelled by barytes during 

 the intense heat required for the decomposition of the mineral. 

 This inconvenience might be, I suppose, prevented by the use 

 of oxide of lead instead of carbonate of barytes. 



The fluoric acid is evidently to be considered as an essen- 

 tial ingredient of mica, and lithion-micas appear to be con- 

 stantly possessed of a large quantity of this acid ; and there 

 seems to exist an insensible passage in that respect from the 

 micas to the talcs, which also deserve to be examined with 

 greater accuracy. I should have wished to try the easily 

 fusible micas from St Gothard before the blow-pipe, but 

 those which I have had an opportunity to examine fused with 

 difficulty, and did not possess the characters of lithion-micas. 

 It does not appear to be an easy task, to separate mechani- 

 cally those portions of the lithion-mica from Chursdorf, that 

 prove to be different from the rest in their optical structure; 

 and I have not at present an apparatus fitted for that purpose. 

 If I should find an opportunity, I shall take the liberty 

 of sending to you a sufficient quantity, that you may exe- 

 cute this separation, and I should be happy to make a careful 

 analysis of both portions. 



I am engaged at present with a comparative inquiry into the 

 volcanic products, and those of what was formerly called the 

 floetz trap formation. It is well known, that muriatic acid has 

 been long since discovered in basalts. This acid seems to be 

 much diffused in these formations. I have, for instance, 

 found it in the prehnite from Dumbarton, and in a great 

 many other minerals of a similar origin. I thought that it 

 might also be an ingredient of Tesselite, hut I could not 



