preserved in Chalcedony, 519" 



with which chemistry furnishes us to distinguish these obscure sub- 

 stances. 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 in- 

 evitably 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 wer6 

 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 pre- 

 sence of carbon in the chalcedony, the two following obvious modes- 

 were adopted. It was previously determined that ail siHcified 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 (chloiite haldogcc or green earth) did not 



