tn the Pyrites. 125 



owing to the fulphur ; without which there is no- 

 thing to be found in the pure iron-pyrites, capable 

 of imparting fuch colour to the iron. That fuch iron- 

 pyrites, whofe fulphur is only mixed with fome arfe- 

 nic, deviates from a yellow more to a white, is un- 

 doubtedly owing to the arfenic. That nothing but 

 the copper heightens the yellow colour of the com- 

 mon iron and fulphur pyrites, and makes it run to a 

 greenifh, is a truth as little to be doubted of. But 

 why a pyrites-copper-cre, containing 20 pounds of 

 copper in the centner, is not fo eminently diftin- 

 guifhable in colour from that which holds only 10 ; 

 and why it is often a difficult matter to know a 

 very rich from a confiderably poor fort, three rea- 

 fons feem affignable: one certainly is, the arfenic, 

 which imparts its high colour to the copper, as is 

 well known from the operation of white-copper- 

 making, and prevents its coming to that degree of 

 yellownefs or greenifhnefs, which copper, with 

 fulphur, is wont to exhibit : and thus fulphur is more 

 copious, nay, as it were, more indifpenfably ne- 

 ceffary in pyrites-copper -ore than in pyrites- iron- ore; 

 as the fandarachy, or ruddy crude fulphur, always 

 procurable from the copper '-pyrites-ores, fufficiently 

 (hews. 



The fecond reafon is the proportion of the ful- 

 phur in refpect to the remaining principles of the 

 pyrites, being what I found pretty uniform and 

 conftant in the pure iron-pyrites- ore ; but in the 

 copper-pyrites ore not only in general, in a fmalJer, 

 but alfo a more variable quantity. 



The third reafon probably may be derived from 

 a circumftance, of which I fhall make fome 

 mention in the following chapter ♦, namely, an 

 unmetallic, crude, undetermined earth \ which, 



as 



