vol.lv.] philosophical transactions. 185 



remains of a less weight than that of the lead originally, its colour is yellow ; 

 with the next increase of weight it passes to orange, which is the colour of 

 glass of lead ; and when the calx is increased more in weight, so that its gravity 

 is become greater than that of the lead originally, it passes into red, the next 

 colour in order. These three colours, succeeding each other in proportion as 

 the gravity of the metal increases, seems to prove that, in this case, the greater 

 density produces the less refrangible colours : and as orange is the colour of this 

 calx, when in a middle degree of weight, between that which is lighter and that 

 which is heavier than the original metal, it appears that orange is the colour 

 natural to lead when its weight is neither much increased nor diminished. 



SILVER. 



The only preparation of silver, which is of any primary colour (except the 

 yellow it imparts to glass, and other vitreous substances, as earths or salts) is 

 luna cornea, which Mr. Boyle says is of a fair yellow, Shaw's Boyle, vol. i, p. 

 255. Physical Essays, Edinburgh, 1754, vol. i, p. 310. 



COPPER and IRON. 



It appears then that all the preparations of gold, lead, and silver, invariably 

 retain the colours peculiar to the order of their densities, and that they are the 

 same with those which they communicate to glass. The two most imperfect 

 metals, copper and iron, being very easily acted on by almost all menstrua, the 

 colours of their solutions, &c. viz. green and blue, are apt to change into each 

 other's order; the copper in some solvents becoming blue, and the iron green; 

 and in other solvents vice versa ; this probably depending on the increase or 

 diminution of their densities. The solutions of copper, in the acids of nitre 

 and sea salt, and in the vegetable acids, are green. But if copper be attenuated 

 by solution in volatile alkalies, it becomes blue. Theophrastus and others have 

 observed, that emeralds are frequently found in copper mines ; and it is probable 

 that they obtain their tinge from that metal. 



I melted some emeralds with twice their weight of salts, and found that they 

 had formed a fine green glass, such as would have been produced from the same 

 quantity of a vitrifiable earth, and about a hundredth part of its weight of 

 copper. — Iron dissolved by the vitriolic acid, is green ; but if further attenuated 

 by a chemical process, it produces that beautiful colour called Prussian blue. 

 Phil. Trans. N° 38. Henckel, Dissert. 6. — A similar blue may be obtained 

 from the iron contained in the ashes of all plants. Henckel, Flor. Sat. chap. 8, 

 parag. 55. — Having exposed a pound of wood ashes in a luted crucible, to a 

 pretty strong fire, for 30 hours, the greatest part of them became tinged blue 

 by the iron contained in them. — A blue may be also extracted from a martial 

 vitriol, by spirit of wine. Henckel, de Appropriatione, chap. 2, parag. 257. 



An instance of a mineral substance changing its colour from green to blue on 



VOL. xii. B B 



