14 
imagination, to take the place of judgment, that, among the several 
systems, of which I shall have occasion to make mention, you, not 
only, will hardly find one on which you can venture to depend ; but 
you will discover, that the majority, so far from possessing even pro- 
bability, rather resemble the fictions of poets, than the reasonings of 
philosophers. 
Yours. 
LETTER III. 
EARLY EXISTENCE OF THESE SUBSTANCES NOTICED BY XENO- 
PHANES, HERODO.TUS, ERATOSTHENES, STRABO, PLINY, OVID, 
PTC SKETCH OF THE HISTORY OF THE SCIENCE. 
A SLIGHT review of the history of what has been hitherto accom- 
plished, in the endeavour to promote and diffuse the knowledge of 
these subjects, cannot, in itself, but be highly interesting. At the 
same time, it will shew the necessity of a work in which a summary 
account is given of the several discoveries and opinions, which have, 
at different periods, been published ; since, by thus placing them by 
each other, in a fair point of view, a proper judgment may be formed 
of their respective merits, and useful truths may be the result of the 
comparisons thus made. 
In the earliest philosophical writings, which have been trans- 
mitted to us, numerous proofs are to be found, that the existence of 
