The Subject Propounded. 7 



as that a fingle drop of a foreign liquor, or the leaft 

 grain of a fait, or fmut, fhall change them : and 

 therefore, to procure fuch as are beautiful, the great- 

 eft nicety and care are to be ufed, as in this cafe, hav- 

 ing to deal with one of the moft delicate fubjecls in 

 nature. Nay, mixtures themfelves come out very 

 different, without the greateft care be had to make 

 perfectly clean a veiTel once ufed, or to employ a 

 new one ; a little common fait, or fal ammoniac, 

 directly adulterating aqua fortis -, the leaft quan- 

 tity of a contrary fait marring an intended fer- 

 mentation ; and a lixivious fait, which has con- 

 ceived only a little acid from the air, becoming 

 unfit for the mercurification of metals, as well 

 as for other procefTes. And in the bufinefs of af- 

 faying, there may fomething, which may ferve 

 to falfify a proof, ftick to the ore-difh, or to the 

 hammer, wherewith the proofs are rubbed, or 

 even to the hare's foot. And in the defulphura- 

 tion of the pyrites, we might, without ufing 

 new retorts, nay, even new receivers, eafily be 

 impofed upon, or at leaft, not fo certainly ob- 

 ferve what pyrites are, and are not, arfenical : 

 as the arfenical impurity may unobfervedly lodge 

 in retorts and receivers, not fufficiently cleared 

 thereof; nor will the fetid arfenical fmell, very 

 foon wear off" ; whence alfo the weight of what 

 pafled over, and what remained behind, cannot 

 always be taken with a neceffary exactnefs -, a cir- 

 cumstance, to be well obferved. 6. Another di- 

 rection of moment, is the accurately entering the 

 feveral experiments into a particular journal ; the 

 numbering the feveral glaffes, boxes and cafes, 

 for holding the different matters to be worked, 

 and the feveral productions procured 5 either for a 

 farther examination, or future ufe. 



And though the pyrites be a mineral fo very 



common, that there is fcarce any earth, ftone, 



B 4 fibre 



