Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/529

Rh with which chemistry furnishes us to distinguish these obscure substances. Chemical analysis is often the only method by which the very doubtful specimens can be ascertained, and if it be necessary to determine precisely all the specimens which bear the semblance of organization, it is the only trial which can be fully depended on, at least it is the only one on which mere chemical mineralogists will be inclined to place any reliance. A considerable experience in the several substances known by the name of Moss agates, combined with some chemical trials on the most leading varieties, and the habits of botanical investigation, may indeed produce that tact in this examination which is well known to mineralogists in other cases; a judgment founded on circumstances so evanescent and minute as to be incapable of communication by words, The inconvenience which follows chemical trials is such as necessarily to preclude its application in many instances, and to render it desirable that accurate descriptions of all the varieties could be formed. The destruction of the specimen, often rare and almost always expensive, must inevitably follow this mode of investigation. I have not therefore subjected to this fiery trial every specimen which I have examined, but have selected such a number of the principal varieties as were sufficient to confirm that evidence which had appeared to result from botanical considerations, and to define in most of the difficult cases the obscure boundary between the real vegetable fibre, and its mimic resemblance, chlorite.

The immediate object of chemical trial being to ascertain the presence of carbon in the chalcedony, the two following obvious modes were adopted. It was previously determined that all silicified wood had the property of blackening and decomposing sulphuric acid, and of giving over carbonic acid on distillation with nitre. It was also ascertained that chlorite (chlorite baldogée or green earth) did not