Xll PREFACE. 



must be expected to impede and interrupt the attempt. 

 The greatest of these appeared to be in the acquisition of 

 a sufficient variety of specimens, to admit of the neces- 

 sary comparison, and to allow the deficiencies of some, 

 so to be made up by others, as to supply a tolerably cor- 

 rect notion of the figure, and even of the nature of the 

 body, which perhaps no single specimen could furnish. 



From this difficulty proceeded, in a great measure, the 

 inability to determine the size of the intended work. 

 The instruction furnished by the specimen which may 

 be possessed to day, may add very little to the know- 

 ledge which we have derived from others; and will there- 

 fore demand but little room for its communication. But 

 the specimen which accident, or industry may throw in 

 our way to-morrow, may furnish that degree of informa- 

 tion, which may lead to a true knowledge of the real 

 nature of the substance under inquiry ; and perhaps to 

 the correction of all previously adopted opinions re- 

 specting it. The new facts which would be thus gained, 

 and the various reasonings and conjectures to which they 

 would lead, would necessarily occasion a wide differ- 

 ence between the space which the article would then re- 

 quire, and that which it had, at first, been expected to 

 occupy. A few such fortunate occurrences, would, it is 

 evident, extend the work much beyond the bounds with- 

 in which it might originally have been proposed to be 



