VOL. XXVI.] PHTLOSOPHICAL TRANSACTIONS. 505 



but the mixture when in fusion took at first a beautiful green colour, and con- 

 tinuing it longer in the focus, it turned bluish. I believe we may ascribe this 

 change of colour to the alcali salts of the glass, acting on the particles of cop- 

 per; for these salts usually draw a green or bluish tincture from this metal. To 

 preserve therefore this red colour of the vitrified copper, when mixed with com- 

 mon glass, I made use of this expedient : I melted in the focus, on a cupel, 

 a piece of copper, and as soon as it began to vitrify I cast upon it some com- 

 mon glass; as soon as the glass was melted, I took them together out of the 

 focus without confounding them ; and when cold, I separated the regulus from 

 the glass as well as possible, picking out of it some pieces of the glass, loaded 

 with some very small red transparent particles of the regulus. This vitrified 

 copper is therefore nothing but copper deprived, by means of heat, of the sul- 

 phureous particles which gave it the form of a metal: for on exposing this vitri- 

 fied copper to the focus upon charcoal, it reassumes in a little time the colour 

 and consistence of melted copper ; and as it cools it fixes into a good, red, 

 malleable copper, as fine and hard as it was before vitrification. From these 

 experiments it follows, that the basis of copper is a red earth susceptible of 

 vitrification : That this earth receives its metallic form from a sulphureous sub- 

 stance, in appearance nowise different from the oil of vegetables or animals : 

 That we may deprive copper of this oil, by holding it long enough in the focus, 

 or by calcining it in the flame of common fire : That charcoal restores again 

 this oily part to copper, and at the same time its metallic form. It appears 

 farther, that the oil of the coal has not so considerable an effect on copper, as 

 on iron. Copper exposed a long time to the focus, on a stone or a cupel, 

 fumes very much, and diminishes very considerably in weight. I do not think 

 that this fume is only the sulphureous part of the metal, the evaporation of 

 which must be insensible ; but I believe that with this oil there is mixed a great 

 deal of the earthy, vitrifiable part of the metal, which the heat of the sun sub- 

 limes and raises in flowers. 



Tin, exposed on coal to the focus of the burning-glass, melts, and emits a 

 gross, white, thick fume, till it is all evaporated. 



By melting tin upon a cupel, in the focus of the glass, it fumes very much, 

 and its surface is covered with a white rarefied calx; on which gradually arises a 

 tuft, or heap of sharp, needle-like, transparent crystalline particles, consisting 

 of a vast number of small points. By continuing to hold this mass in the focus 

 on the stone, these crystals at length cease fuming, and remain fixed, while the 

 stone melts and vitrifies. 



I took calx of tin, (which is tin reduced to a grey powder by means of fire, 

 which has taken away by calcination great part of its oily substance) and ex- 



VOL. V. 3 T 



