184 PHILOSOPHICAL TRANSACTIONS. [aNNO 1765. 



in a solution of alkaline salt, becomes red on being exposed to a slight heat. 

 Lewis, history of gold, p. 108. — 2. The same colour is produced when this 

 precipitate of gold is ground with oil of vitriol, or spirit of sulphur; or if it be 

 mixed with sulphur, and the sulphur burnt away. Junker, tab, 33, p. 85Q. — 

 3. The smoking spirit of Libavius, mixed with gold, and afterwards drawn off 

 from it by distillation, changes its colour to a blood red. Sol sine veste, exp. 

 IQ. Junker, tab. 33, p. 86 1. — 4. Gold is reduced into a red powder, by amal- 

 gamation with mercury, and exposing it for a considerable time to a slow heat. 

 Boyle's Abridg. vol. -2, p. 77. Junker, tab. 3Q, p. g87. — 5. If 6 parts of 

 antimony be fused with one of gold, and the antimony driven off by the blast, 

 a red powder of gold is left behind. Cassius de Auro, cap. 10. — 6. If gold 

 leaf be cemented and ground with decrepitated salt, hartshorn, pumice, or chalk, 

 and exposed to a proper heat, the metal becomes red, and may be precipitated 

 from a solution of those substances in a red powder. Junker, tab. 33, p. 854. 

 Lewis's history of gold, p. 74. Sol sine veste, cap. 6. — 7. A red tincture may 

 be prepared from gold by several methods mentioned by Libavius, Alchem. lib. 

 2, p. 130. Junker, tab. 33, p. 868. — 8. A solution of gold in aqua regia, 

 prepared from sal ammoniac, may be sublimed of a blood red colour. The same 

 is effected by dissolving the calx, or crocus of gold, in other menstrua. Lewis's 

 history of gold, p. 100. Junker, tab. 33, p. 857- — 9- A solution of gold in 

 aqua regia, evaporated properly, affords crystals of a bright red colour. Cassius 

 de Auro, p. lOQ. Junker, tab. 33, p. 862, 868. Lewis's history of gold, p. 

 QQ, — 10. Aurum fulminans moistened with water, has been found to tinge 

 gems deeply of a fine red. Phil. Trans. N° 179- — H- A solution of gold 

 tinges ivory, cotton, the skin, and other substances red. 



Rubies being frequently found in gold mines, it is very probable that they re- 

 ceive their colour from that metal ; and from this circumstance, before the ex- 

 periment had been made, Libavius rightly conjectured that a solution of gold 

 would communicate a ruby colour to glass. Libavii Alch. p. 88. — It does not 

 appear that, excepting the colour natural to gold in its entire state, any other 

 than red can be obtained from preparations of this metal : it is from this colour 

 which gold assumes whenever its metallic brightness is destroyed, that writers in 

 chemistry call it leo ruber. 



LEAD. 



The only coloured preparation of lead, is that produced by calcination in the 

 furnace. The first of the primary colours produced by this process is yellow, 

 the calx passing from that colour through orange into red. It is remarkable, 

 that though in the calcination previous to the reverberatory heat in which these 

 colours are produced, the lead is diminished in weight ; yet in the reverberatory 

 fire it gains considerably, and in proportion to that increase of gravity, it passed 

 from the more refrangible to the less refrangible colours ; so that while the calx 



