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XX. Some abſolutely affirm, that all Stones will melt in the Fire except Marble, which by burning is reduced to Lime: But this is ſaying abſolutely, and of all, what ought only to be ſaid in general, and of the greater Number.
this Change of Colour, is not leſs eaſily and certainly diſcoverable from the Colour itſelf; than what has influenced the Shape, from the Form. If Lead has furniſhed the metalline Particles, the Spar is yellow; if Iron, red; if Tin, black; if Copper, it is either greeniſh or blueiſh, according to the Quality of the Menſtruum Nature has furniſhed for diſſolving the Particles of that Metal, and bringing them into a State of mixing in the Concretion; for Acids and Alkalis both diſſolve Copper, but with this Difference of Colour, that the Solution with an Acid is green, and that with an Alkali is blue.
Though this Author was perfectly right, therefore, in his Opinion of theſe Subſtances partaking of the Nature of the Metals among which they were found; he errs in imagining that they are fuſible, and melt with thoſe Metals. He may very well, however, be pardoned in this, ſince it has been an Error which many later Authors, who had more Opportunities of informing themſelves of the Truth than he can reaſonably be ſuppoſed to have had, have alſo fallen into; nay, and many who imagine they underſtand theſe Things very well, from the conſtant Uſe of it in fluxing the Ores of Metals, believe the ſame of it even yet. This is however an abſolutely erroneous Opinion, for Spar is not fuſible, but calcines in the Fires uſed for melting the Ores of Metals. The Uſe it is of, in the fuſing them is this: Thoſe Ores are frequently clogged and loaded with Sulphurs, which make them very difficult of Fuſion; and the Calx of Spar is of the ſame Uſe in that Caſe, that Lime, or any other fixed Alkali would be: That is, it abſorbs thoſe Sulphurs; and by that means deſtroying what would impede the Fuſion of the Ore, does in ſome Senſe aſſiſt its melting; but no one, who ever ſaw the Fuſion of Ore with its Spar about it, ever yet obſerved the leaſt Particle of that to melt.
The Pyritæ and Molares, as many Kinds of them were originally called, are no more capable of Fuſion in the Fire than the Spars. They are Maſſes of mineral, ſaline, and ſulphu-
