Metals. Metals are elementary or undecompounded bodies, which are distinguished by their weight, lustre, fusibility, power of conducting heat, electricity, etc.. (see Nos. 3349 (Properties of Metals) to 3357 inclusive), and the numerous compounds which they furnish by combination with one another, and with other bodies. When their solutions are decomposed by a galvanic current, the metals always appear at the electro-negative surface, and are hence termed electro-positive bodies.

3187. Assaying

3187. Assaying. The method of determining the quantity of pure gold and silver in the alloys of these metals. -This art requires great skill and experience in its performance ; and, from the costliness of the precious metals, is of the utmost importance. A downward draught furnace of any shape and size may be employed, provided it will afford a sufficient heat, and allow the introduction of the muffle. The muffle is a pot made of clay, and furnished with an opening at its end, to admit the introduction of the cupels, and to allow of inspection of the process. It is placed on the muffle-plate, by which it is introduced into the furnace. The cupel is a sort of shallow crucible, made of bone ashes or burnt bones. At the British mint the cupels are made of the calcined cores of ox-horns. The powder is slightly moistened with water, and a circular steel mould is filled therewith, and after being pressed down tight, is finished off with a rammer, having a convex face of polished steel, which is struck forcibly with a mallet, until the mass becomes sufficiently hard and adherent. The cupel is then carefully removed, and exposed in the air to dry, which usually takes from 14 to 21 days. The muffle, with the cupels properly arranged, being placed in the furnace, the latter is filled up with charcoal, and lighted at the top by placing a few pieces, heated to whiteness, on last. When the cupels have been exposed for half an hour, and have become white by heat, the lead is put into them by means of a pair of tongs, and as soon as this becomes thoroughly red and circulating, as it is called, the metal to be assayed, wrapped in a small piece of paper, is added, and the fire kept up strongly until the metal enters the lead, and circulates well, when the heat may be slightly diminished, and so regulated that the assay shall appear convex and ardent, while the cupel is less red - that the undulations shall circulate in all directions, and that the middle of the metal shall appear smooth, surrounded with a small circle of litharge, which is being continually absorbed by the cupel. This treatment must be continued until the metal becomes bright and shining, or is said to "lighten;''' after which certain prismatic colors, or rainbow hues, suddenly flash across the globules, and undulate and cross each other, and the latter metal soon after appears very brilliant and clear, and at length becomes fixed and solid. This is called the "brightening," and shows that the separation is ended. In conducting this process, all the materials used must be accurately weighed, especially the weight of the alloy before cupellation, and the resulting button of pure metal. The difference gives the quantity of alloy. The preceding general description of the process of cupellation will render the following articles intelligible, without again entering into the minutiae of the operation. An assay is thought to be good when the bead is of a round form, with its upper surface brilliant, its lower one granular and dead-white, and when it separates readily from the cupel. When the surface of the bead is dull and flat, it shows that too much heat has been employed ; and if the metal be silver, some may have been lost in the process, by fuming or absorption. When the bead is spongy, and of various colors, and scales of litharge still remain on the cupel, and the metal adheres strongly to the latter, too little heat has been used, and the button still retains some lead. To remedy this, the heat should be raised, and a little powdered charcoal, or a few small pieces of paper, thrown into the cupel, until the metal again begins to circulate freely. It is necessary that the lead employed in the process of cupellation should be perfectly pure. It ought, therefore, to bo procured by reducing refined litharge. (Cooley.)