259 



some subtle matter id lodged in the pores of the lime 

 (perhaps many of those particles of fire, whereby the 

 stone was reduced to dust) which when the water 

 insinuates into (hose pores, occasion the same kind of 

 ebullition, as if it was poured on any other burning 

 substance. 



15. Most precious stones are transparent^and strike 

 the eye with vivid and various colours. Probably 

 they were once fluid bodies, which while in that state 

 were mixed with metallic or mineral juices. Their 

 transparency likewise makes this probable, and so 

 does their outward configuration. For many bodies 

 hardening into solids shoot into crystals, just as is 

 observed of several kinds of precious stones : and 

 to this their -inward structure answers. For in many 

 we may observe the thin plates or coats one over 

 the other, just as we see in those mineral substances, 

 which were once fluicR Their colours might be 

 owing to some mineral juice or exhalation, which 

 tinctured them before their pores were fully closed. 

 This is the more probable, because many gems lose 

 their colour, if they lie long in the fire, and because 

 generally coloured gems are found over metallic or 

 mineral veins. 



Dr. Boerhaare takes crystal to be the basis of all 

 precious stones, which assume this or that colour, 

 from the metallic or mineral steams mixed with the pri. 

 mitive crystalline matter. But how is crystal itselfform. 

 ed ? An Italian writer gives aparticular account of lijis. 

 In the Val Sabbia, says he, I observed some parts of 

 a meadow bare of all herbs. Here, and no where else 

 thereabouts the crystals are generated. And when, 

 ever there is a serene and dewy sky, if all the chrys- 

 tals that can be found over night, are taken away, 

 others will be found in the same place in the morning. 

 Having observed, there is no sign of any mineral 

 steam near, I conclude they are produced by steams of 

 nitre. These may at the same time hinder vegeta- 

 tion in those places^ and coagulate the dew that falls 



