The dry assay of nickel (cobalt being at the same time determined) is based on the formation of a speise which will carry the cobalt, nickel, copper, and some of the iron of the ore in combination with arsenic. A speise of this kind, fused and exposed at a red heat to air, first loses arsenide of iron by oxidation. It is only when the iron has been oxidised that the arsenide of cobalt begins to be attacked; and when the removal of the cobalt is complete, the nickel commences to pass into the slag, the copper being left till last. The changes are rendered evident by fusion in contact with borax. The process is as follows:--Weigh up 5 grams of the ore, and calcine thoroughly on a roasting dish in the muffle. Rub up with some anthracite, and re-roast. Mix intimately with from 3 to 5 grams of metallic arsenic, and heat in a small covered clay crucible at dull redness in a muffle until no more fumes of arsenic come off (about 15 minutes). Take out the crucible, and inject a mixture of 20 grams of carbonate of soda, 5 grams of flour, and 2 grams of fused borax. Place in the wind furnace, and raise the temperature gradually until the charge is in a state of tranquil fusion. Pour; when cold, detach the button of speise, and weigh. Weigh out carefully a portion of about 1 gram of it. Place a shallow clay dish in the muffle, and heat it to bright redness; then add about 1.5 gram of borax glass wrapped in a piece of tissue paper; when this has fused, drop the piece of speise into it. Close the muffle until the speise has melted, which should be almost at once. The arsenide of iron will oxidise first, and when this has ceased the surface of the button brightens. Remove it from the muffle, and quench in water as soon as the button has solidified. The borax should be coloured slightly blue. Weigh: the loss is the arsenide of iron. Repeat the operation with the weighed button on another dish, using rather less borax. Continue the scorification until a film, green when cold, floating on the surface of the button shows that the nickel is beginning to oxidise. Cool, separate, and weigh the button as before. The loss is the arsenide of cobalt. If copper is absent, the speise is now arsenide of nickel. The weight of nickel corresponding to the arsenide got is calculated by multiplying by 0.607; and, similarly, the weight of the cobalt is ascertained by multiplying the loss in the last scorification by 0.615.[71] It must be remembered that the nickel and cobalt so obtained are derived from a fraction only of the speise yielded by the ore taken, so that the results must be multiplied by the weight of the whole of the speise, and divided by the weight of the fragment used in the determination. As an example, suppose 5 grams of ore gave 3.3 grams of speise, and 1.1 gram of this gave 0.8 gram of nickel arsenide. Then-- 0.8×0.607 = 0.4856 gram of nickel 0.4856×3.3/1.1 = 1.456 gram of nickel