( 2^2 ) 



iblmd to be compored of alumlne> dlcx^ 

 and a very fmall quantity of lime. It is 

 ufuaUy refractory to the fire of a glafs 

 furnace ; but fornetimes v;^ill fufe when it 

 contains, a rather larger proportion of lime. 

 Of this kind of earth I made fome fmall 

 fpheres and cubes, which I left four or 

 five hours in the furnace ; when the vitri« 

 fication had penetrated to the depth of two 

 thirds of a line, and in feme even a whole 

 lin?, and the central part had become femi- 

 vttrecus. 



Similar eftedis take place In lavas. Their 

 fu-fface afliimes an enamel or vitreous 

 afpecl, which in the internal part is lefa 

 apparent. In fome, however, we find 

 points which have become vitreous, though 

 thefe lavas before did riot contain a fmgle 

 atom of glafs. In general, they lofe that 

 grain and texture, which indicate the 

 primitive rocks whence they were derived, 

 and they all, or nearly all, may be called 

 vitreous. 



