S28 Mr, Delaval on the Cauje of the 



not admit the palTage of the lefs refrangible rays, 

 they are not feparated from the more refrangible 

 rays, but are conjointly intercepted and refleiled, 

 by the white particles, which are contiguous to 

 the anterior furface, and therefore the mafs 

 appears white. 



When a bright light, fuch as the fun, or the 

 fiame of a candle, is viewed through a thick 

 mafs of fuch glafs, fo many of the more refran- 

 gible rays are intercepted, that the tranfmitted 

 rays compound a Red Colour. 



The glafs, of which bottles are ufually made, 

 affords another inftance of a femipellucid fub- 

 ftance, which exhibits a blue colour by incident, 

 and a yellow or orange colour, by tranfmitted 

 light. 



This fort of glafs, when in its perfcd (late, 

 tranfmits a green colour, which it owes to the 

 iron contained both in the vegetable afhes, and 

 in the fea fand, which enter into its compofition.. 



The flux, by which the materials of this glafs 

 are fufed, is fo loofely conneded with them, 

 that it is difengaged by expofure to a red heat. 

 In proportion as the glafs is decompofed, by the 

 lofs of its flux, the earthy particles, which are 

 lio longer diflblved, become Opake. If the 

 glafs be withdrawn from the fire, whilft only 

 a fmall part of the flux is difengaged, the rnafs 

 conflfts of Tranfparent Glafs, throughout which 

 the opakc earthy particles are diflVminated. 



If 



