Ve-rmanent ColoHYS of Op<ike Bodies. b.i^ 



If the glafs be furrounded with white fand, or 

 other earthy fubftances, and expofed to a greater 

 heat, it entirely lofes its tranfparency, and tex- 

 ture, and aflTumes the appearance of porcelain 

 or earthen ware, as M. Reaumur firft obfervcd. 

 I have continued this procefs fo far, that the 

 whole of the flux was difengaged from the other 

 materials of the glafs. 



After having expofed, in a ftrong, continued 

 heat, the neck of a bottle, filled, and furrounded, 

 with white fand, I obferved that all the flux had 

 quitted the glafs, and united with the fand, 

 which had been in contact with its external, and 

 internal, furfaces. For the fnace, which had 

 contained the glafs, was occupied by a loofe 

 fandy powder, and on each fide of that fpace, • 

 the white fand was united into a firm, confiftenr, 

 mafs, which reprefenced a mould, whofe cavity 

 was of the exaft form, and fize, of the neck of 

 the bottle, from whofe flux it received its (hape, 

 9nd confiftence. 



When the glafs, from the degree of heat, to 

 which it has been expofed, is reduced to fuch 

 a flate, that it confifts of a Tranfparent Medium, 

 mixed with Opake White Particles, it exhibits 

 a blue colour by incident light, and a yellow, 

 or orange colour, by tranfmitted light. For' 

 the more refrangible rays being interc'^pted and 

 refiedled by the white earthy particles, pafs back 

 through the fpace, which intervenes between 



Qv3 thofe 



