2J0 The fpecific Gravity of 



dies, ftand related in fpecific gravity ; and by this 

 means not only learn to difcover the nature of the 

 refpe&ive bodies : for inftance, that arfenic ap- 

 proaches extremely near to the nature of metals. 

 We are alfo hereby furnifhed with a teft to judge 

 of the relation between this and other bodies in the 

 firft table, which may be fafely depended on : for 

 thus, a fulphur, the more arfenical it is, the hea- 

 vier; an arfenic, the more fulphureous, lighter 

 than a white cryftalline arfenic ; and an orpiment 

 muft contain more of arfenic than of fulphur ; a 

 •pyrites, and an arfenic pyrites, the heavier, the more 

 arfenical, &V. Only it is well to be obferved, ( i.) 

 that the body to be examined have not the leaft fo- 

 reign admixture, as ftone, earth, or other ore ad- 

 hering; (2.) be not fibrous and porous, but clofe 

 or denfe, for the air not to lodge entangled in it, or, 

 not with (landing, being capable of being exhaufled, 

 as both thefe circumftances render the proof falla- 

 cious. This, for inftance, is the reafon why crude 

 fulphur appears lighter than purified fulphur; 

 whereas crude fulphur, when of a grey caft, as this 

 was, and generally is, holds a little arfenic, and 

 though never fo little, it muft at leaft preponderate 

 purified fulphur. Further, foflile fulphur weighs 

 more than purified fulphur, the latter not being fo 

 clofe or denfe as the former ; whence that fort melt- 

 ed once more over, and thereby becoming clofer, is 

 heavier than the purified. Yet clofenefs, or denfity 

 and gravity go not always hand in hand ; flint, for 

 inftance, is lighter than quartz, and this much 

 lighter than fpath: white cryftalline arfenic is 

 much lighter than foflile fly-ftone, which fublimes 

 entirely in form of a white arfenic, whence the 

 cryftalline arfenic muft either have acquired or loft 

 fomething, which muft needs have made it lighter. 

 Nor need we wonder that yellow, or copper pyrites, 



is 



