I have before mentioned the great affinity betwixt metallic and inflammable ſubſtances (§ 133). Zinc and arſenic stand, as it were, upon the borders betwixt them; for these, in proper circumſtances, burn with a very evident flame. All the metallic ſubſtances contain phlogiſton, and when, to a certain degree, deprived of it, fall into a powder like an earth; but their attractions for phlogiſton are different. Moſt of them, when melted in a common way, and expoſed to the air, have an earthy cruſt formed upon the ſurface, which cannot again be reduced to metal without the addition of ſome inflammable matter. The baſe metals, eleven in number, have this property: but the noble metals, platina, gold and ſilver, are ſo firmly connected to the phlogiſton, that they never calcine under fuſion, however long continued; and after being changed into a calx in the liquid way, when melted in the fire, they re-aſſume their metallic form, without any other phlogiſton than what is contained in the matter of heat.

Quickſilver holds a kind of middle place; for, like the baſe metals, it may be calcined, though not readily; and like the noble ones, it may be reduced by heat alone.

I have placed each diviſion of the metals in the order of their specific gravities.