VOL. LV.] PHILOSOPHICAL TRANSACTIONS. 181 



several authors, as a composition proper, without the addition of any other in- 

 gredient, for imitating the hyacinth. 



SILVER. 



Yellow is the only colour which silver, the metal next to lead in density, can 

 by any preparation be made to impart to glass. 



1 . Without insisting on what some chymists affirm, that silver, on being cal- 

 cined and exposed to a violent fire for a long time, was partly reduced to a 

 yellow glass. 2. I have often given that colour by moistening the surface of 

 the glass with a solution of silver, and afterwards making it red hot. 3. If 

 silver be calcined with sulphur, it readily communicates a yellow colour to glass. 

 4. Having carefully purified an ounce of silver, I kept it in fusion some hours, 

 with a small quantity of glass, and found that the glass, when cold, had formed 

 a beautiful yellow enamel on the surface of the silver. 5. Leaf silver, laid on 

 reti hot glass, tinges it yellow. 



When we meet with authors, who mention a blue or greenish colour commu- 

 nicated by silver, the cause must have been, that the silver used in such pro- 

 cesses was mixed with copper, as it generally is, when it is not carefully purified. 

 I have always found, that silver purified by the test retained s(5 much copper, 

 that when melted several times with nitre and borax, it imparted a green tinge 

 at the 1st and 2d melting, though afterwards no such colour was obtainable 

 from it. 



COPPER. 



Green is the only colour which copper, the metal next to silver in density, 

 communicates to glass, when melted with it in a sufficient heat, without any 

 additional ingredient : Thus 1 . By grinding crystal glass in a copper mortar, and 

 afterwards melting it, it becomes green. 2. Copper calcined per se in a furnace. 

 3. Copper calcined with sulphur ; and 4. Scales beaten off from red hot copper 

 plates, mixed with glass, equally impart a green colour to it. 



It is indifferent in what manner the copper is prepared, in order to tinge the 

 glass green, provided it be exposed, without any other ingredient, to a sufficient 

 degree of heat. I have frequently produced a fine green from copper filings 

 unprepared. If a quantity of salts be added in the preparation, they will, by 

 attenuating the mixture, and consequently lessening its specific gravity, make 

 the glass incline to blue, the colour next in order ; but this happens only when 

 the fire is moderate ; for in a greater degree of heat, the redundant salts, even 

 those of the most fixed nature, are expelled. 



It is true, that copper is mentioned by some writers, as an ingredient in red 

 glass and enamel : but the red, which is the colour of the metal not dissolved or 

 mixed with the glass, remains only while the composition is exposed to such a 

 degree of heat as is too small to melt and incorporate it ; for, if it be suffered 



