ly 1 A4r. Delaval on the Caufe of tht 



All the experiments, which have been made 

 with tranfparent coloured glajjes, fliew that they 

 appear vividly coloured, when they are of fuch a 

 thinnefs, and are tinged fo dilutely, that light is 

 tranfmitted through them. But v/hen they are 

 in larger mafles, and the tinging matter is more 

 denfely diffufed through them, they are l^lack. 

 For, thefe as well as the tranfparent coloured li- 

 quors do not exhibit colours by reflect ion ^ but by tranf^ 

 miffion only. 



I fhall here inftance fome trials, which I made 

 for the purpofe of obferving the proportions 

 of tinging matter, which produce colour, or 

 blacknefs. 



I tinged glafs green, by adding to it one fix- 

 tieth part of its weight of copper. It is indiffer- 

 ent whether the copper, ufed in this procefs, be 

 calcined, or applied in its metallic ftate. 



1 made a blue glafs, by the addition of zafFre, 

 2 purple glafs by manganefe, a red glafs by gold, 

 and yellow glafles by filver, and by calcined iron. 

 I alfo made a yellow glafs, fimilar to a topaz, 

 by the admixture of a fmall quantity of pulve- 

 jfized charcoal : and the fame colour was impart- 

 ed by wheat flower, by rofin, and by feveral other 

 inflammable matters. 



From the mafles of tranfparent coloured glafs, 

 which were thus compofed, fmall pieces were 

 feparated, and ground by a lapidary, which re.- 

 fembled gems of their refpeflive colours. 



Having 



