6 Tbe Subject Propounded. 



on the breaking of a veflfel, or other failure in. 

 the courfe of the procefs. 4. I have ufed the 

 greateft care in chufing and feparating the feveral 

 matters employed -, a circumftance of no fmall 

 importance -, for, not to mention the various forts 

 of folvents, falts, &c. (which notwithftanding 

 are to be well examined in refpect to their good- 

 nefs ; fo as we may be able to diftinguifh, for 

 inftance, between alcali and alcali, fait of tartar 

 and potafh) we mull be careful, knowingly not to 

 employ for proof any communicative additions, but 

 the bare pyrites alone. The feveral famples employ- 

 ed, were fuch as I either originally procured myfelf, 

 or had from perfons of credit and veracity. Theie 

 I carefully examined and feparated from each fo- 

 reign admixture : which was found needful, even 

 when externally they feemed to give no fuch fuf- 

 picion \ as is plain from the fteely or clofe py- 

 rites of Pretzfcendorff ; having experienced the 

 feemingly pureft fort of them, to be not only in- 

 ternally full of Mures, or fibres, but thefe fifTures 

 to be often mock-leady, and thus apt to mifguide 

 or at leaft to puzzle the moft careful operator. And 

 the pyrites -kidnies of Franckenberg, how clofe 

 foever they may feem externally, and how fimilar 

 foever, internally, yet fhew not only a black 

 mock-leady impurity, but alfo fuch interfperfed 

 pieces of copper -pyrites, as are evidently diftin- 

 guifhable from the reft of the fample, namely, 

 the genuine iron-pyrites. 5. A no lefs degree 

 of care I ufed with regard to the veffels and 

 inftruments, employed in making the experi- 

 ments : a caution of greater importance than the 

 unexperienced may imagine. The colours ma- 

 nifefting themfelves in experiments, fhew much 

 of the internal nature of bodies, and their pro- 

 ductions : or, at leaft, how one fort comes to 

 differ from another -, and are of fo peculiar a nature, 



as 



