confined : and might necessarily occasion an extension 

 of the work beyond those limits, within which the au- 

 thor had bound himself to confine it. On this account, 

 it was resolved to leave the size of the work to be deter- 

 mined, by the quantity of additional information which 

 might be furnished, by the accession of new materials ; 

 taking care, at the same time, to guard against the ad- 

 mission of uninteresting and irrelevant matter. 







How far the attempt to form the present volume on 

 these principles, and on this model, has succeeded, the 

 author leaves to the judgment of the candid: assuring 

 them, that in the prosecution of the work, no unwar- 

 rantable protraction of it shall be admitted. He is very 

 desirous to complete the work in a third volume; but 

 when he considers the multitude of subjects which 

 must be examined, he finds himself unable to pledge 

 himself to the observance of the limits which would be 

 thus prescribed. 



Anxious to obtain, and to communicate, the fullest 

 information on the various subjects of his inquiry, he 

 will be much obliged by any specimens or observa- 

 tions, illustrative of the nature of the different substances 

 which have been, or which remain to be, examined. Any 

 such communications he will with pleasure place before 



