4 Dr John Davy on the 



These two results (not to revert to others) seem to shew, 

 and it appears to me in a satisfactory manner, that silica 

 may be, and is kept in solution by carbonic acid. If this be 

 admitted, may it not aid in the explanation of much that is 

 obscure in regard to the structure of certain rocky strata, 

 especially of aggregates, in which the cementing principle 

 appears to be silica % May it not also aid us in explaining 

 the manner in which certain fossils are formed, such as are, 

 as it were, casts, the petrifying matter of which is silica % 



Hitherto a fixed alkali has been received as the solvent of 

 silica, in mineral waters. That it is a solvent in its caustic 

 state, I apprehend, must be granted ; — but I am not aware 

 of any facts or analogies tending to prove that it is also such 

 a solvent when saturated with carbonic acid. And as, in 

 every instance in which I have had an opportunity of ex- 

 amining waters containing silica held in solution, I have 

 found carbonic acid, it seems to me natural to infer, that it 

 Avith water is the solvent. As regards this view, it is desir- 

 able that the waters, most remarkably siliceous, should be 

 re-examined ; especially the water of the Great Geyser in 

 Iceland, the petrifying effect of which, as it falls on the plants 

 in the neighbourhood of the fountain is so remarkable, and 

 so obscure at present, but which would cease to be so should 

 it be proved that the deposition of silica in this instance is 

 connected not only with a great and sudden reduction of 

 temperature, but also with the rapid escape of carbonic acid, 

 — the generation and evolution of which may perhaps be con- 

 cerned in the production of some of the peculiar phenomena 

 of this extraordinary spring. 



The island of Nevis, like Iceland, is volcanic. Some of its 

 rocks are highly crystalline, and abound in felspar, or in what 

 appears to be felspar. Clay, too, is abundant in the island. 

 Probably the potash which occurs in the mineral water is 

 derived from the decomposition of felspar, or of analogous 

 minerals, aided by heat, which expels carbonic acid from com- 

 pounds containing it ; and this acting on the silica liberated 

 from the decomposing minerals, may, with water, effect ita 

 solution : — I say liberated, because we know that certain 

 substances, perhaps all, then most readily unite with others 



