( iS9 ) 



nace, both of lavas and primordial rocks 

 and ftones, thofe likewife from which pu- 

 mices are moft frequently produced, as pe- 

 troiil^ces and feltfpars, I never obtained a 

 product which could be faid to poffefs all 

 the charad;eriftics of pumice. It has al- 

 ways been either a glafs, an enamel, or 

 fcoris ; nor do I remember to have read 

 or heard, that among the innumerable ex- 

 periments which chemifts have made upon 

 earths with fire, the refult ever was a true 

 pumice : and though in lime-furnaces we 

 may frequently obferve a change of certain 

 ftones into glafs ; yet they never produce 

 lavas fimilar to the volcanic ; and equally 

 incapable are they to form pumices. It 

 cannot be alleged that the fire of our fur- 

 naces is too ftrong to induce that flight de- 

 gree of vitrification which characterizes 

 pumices ; fince, when I have ufed a more 

 moderate fire, the fubfi:ances on w^hich I 

 made my experim,ents have either not 

 melted, or been more or lefs vitrified. 



I (hall conclude thefe obfervatlons on the 



pumices 



