14 



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 

 probability, rather resemble the fictions of poets, than the reason- 

 ings of philosophers. 



Yours. 



LETTER III. 



EARLY EXISTENCE OF THESE SUBSTANCES NOTICED BY XENO- 



PHANES, HERODOTUS, ERATOSTHENES, STRABO, PLINY, OVID, 

 &C 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 show the necessity of such a work, as is here 

 attempted ; in which a summary account is intended to be given 

 of the several discoveries and opinions, which have, at different pe- 

 riods, been published ; since, by thus placing them by each other, 

 in a fair point of view, a proper judgment may be framed of their 

 respective merits, and useful truths may be the result of the com- 

 parisons thus made. 



In the earliest philosophical writings, which have been transmit-* 

 ted to us, numerous proofs are to be found, that the existence of 



