and on the Formation of Agaric Mineral, 107 



chalk has been deposited from solutions, and that the fluid 

 from which it has been precipitated contained some matter 

 that had been organized dissolved in it, it may help to ac- 

 count for the peculiar form of chalk, which, in its structure, 

 is an approach to agaric mineral. This idea I venture to 

 propose as a conjecture on an obscure subject. I have en- 

 deavoured to test it by precipitating lime, as a carbonate, 

 from solutions with which an infusion of some vegetable or 

 animal substance had been mixed ; but the results have been 

 ambiguous, — some apparently in accordance with the notion, 

 some contrary to it. Some facts, which are well known, may 

 be adduced as seeming to favour it. The cleanest sand, most 

 free from impurities, is required to make the strongest mor- 

 tar. The carbonate of lime of sea-water is not found to be 

 deposited on marine vegetables, nor on shores where there 

 are impurities brought down from the land by streams — 

 a happy circumstance, allowing of their dispersion — but on 

 coasts, and in situations where the water is clearest, and 

 the sand cleanest, and where the waves break with the 

 greatest violence, — all circumstances, I believe, favourable, 

 both to the deposition of carbonate of lime, by the separation 

 of its solvent carbonic acid, and to its crystallization, and 

 thereby to its cementing quality. In the paper already re- 

 ferred to, in which some cements of carbonate of lime, of un- 

 usual purity, found in ancient buildings, are mentioned, of 

 chalk -like softness and texture, the opinion is hazarded, that 

 that chalk-like condition may have been owing to the purity 

 of the material ; and considering that calcareous mortars, 

 without sand, consist, in the first instance, of hydrate of 

 lime in its solid state, its particles being granular (so they 

 appear under the microscope), may favour an amorphous 

 rather than a crystalline formation of carbonate of lime ; 

 whilst the reverse may be the case, and would seem to be so, 

 when carbonate of lime is deposited, pure and unmixed, from 

 a solution. And the traces of organized matter which are 

 found commonly in chalk, either distinctly in the remains of 

 the silicious skeletons of infusoria, or, indistinctly, in its ac- 

 quiring a greyish hue when heated to a certain degree, which 



