xn 
PREFACE. 
must be expected to impede and interrupt the attempt. 
The greatest of these appeared to he 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 
require, 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 
within which it might originally have been proposed to be 
