Fufibility. 53 is inteuded to be explored. That the fufion of feveral earths and mixtures, effayed both by Mr. Achard and Mr. D’Arcet, arofe from this caufe, can fcarcely be doubted. Mr. Achard found a mixture of two parts calcareous earths and one part magnefia vitrifiable yet Mr. Lavoifier, who tried their fufibility in a much higher heat, found them perfe&ly infufible in every propor- tion •f', and I found a mixture of thofe fubftances, in the proportion mentioned by Mr. Achard, to pafs through the crucible. The length of time, during which they are expofed to heat, necef- farily renders them liable to this accident, which is avoided by the brifk and fudden mode ol heat- ing them which takes place in a forge. Yet it muft be allowed that aftion on the crucible can only take place where calx, or magnefia, or bary- tes, are employed, either alone or with a finall Proportion of argill and filex; and conlequently the fufibility obierved by Mr. Achard in other cafes is above fufpicion. In my experiments the lime was formed of the pureft Carrara marble. The magnefia was Hen- ry’s calcined, whofe purity I never had caufe to fufpeft. The filex was pure tranlparent cryftal, heated to rednefs, quenched in diftilled water, and then reduced to impalpable powder in a glafs mortar, which it did not corrode. The barytes was the natural mild barytes. Of the calces of iron, I employed two forts; one thoroughly cal cined by lolution in the nitrous acid, and expul- fion of the acid in a red heat; the other com- nion ruft, from which much of its water and * Mem. Beil. 1780. 65. f Mem. Paris 1783. 595, and 599. F. 3 fixed