PREFACE. 
Xlll 
confined : and might necessarily occasion an extension 
of the work beyond those limits within which the author 
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 
