Increase the fire by degrees, as more and more litharge is formed and absorbed. If the Regulus examined by this assay contain no Silver, you will see it turn wholly into scoria, and at last disappear. When it contains Silver, and the quantity of Lead is much diminished, you will perceive little vivid irises, or beautiful rain-bow colours, shooting swiftly along its surface, and crossing each other in many different directions. At last, when all the Lead is destroyed, the thin dark skin, that is continually protruded by the Lead while it is turning into litharge, and which hitherto covered the Silver, suddenly disappears; and, if at this moment the fire happen not to be strong enough to keep the Silver in fusion, the surface of that metal will at once dart out a dazzling splendour: but, if the fire be strong enough to keep the Silver in fusion, though freed from all mixture of Lead, this change of colour, which is called its _fulguration_, will not be so perceptible, and the Silver will appear like a bead of fire. These phenomena shew that the operation is finished. But the cupel must still be left a minute or two under the muffle, and then drawn slowly out with the iron hook towards the door of the furnace. When the Silver is so cooled as to be but moderately red, you may take the cupel from under the muffle with your little tongs, and in the middle of its cavity you will find an exceeding white bead of Silver, the lower part whereof will be unequal, and full of little pits. _OBSERVATIONS._ The Regulus obtained by the former process consists altogether of the Silver contained in the ore, alloyed with the other metals that happened to be mixed therewith in its mineral state, and a good deal of the Lead that was added to precipitate the Silver. The operation of the cupel may be considered as the sequel of that process, being intended only to reduce into a scoria whatever is not Gold or Silver. Lead being of all metals that which vitrifies the most easily, which most promotes the vitrification of the rest, and the only one which, when vitrified, penetrates the cupel, and carries along with it the other metals which it hath vitrified, is consequently the fittest for that purpose. We shall see in its place, that Bismuth hath the same properties with Lead, and may be substituted for it in this operation.