Tin precipitates gold in the form of a purple powder, called the _powder of Cassius_, from the inventor of it, and is used in enamels, or the glassy coating which is given to metals by heat. Gold unites with most of the metals, especially with mercury, and these mixtures are called _amalgams_. In gilding, the amalgam is applied to the surface of the metal to be gilded, and the mercury is driven off by heat, leaving the gold attached to the surface. Gold mixed with iron, makes it harder, for the purpose of cutting instruments. To separate gold from the imperfect metals, such as copper, &c. it is mixed with lead, and then exposed to a strong heat, which calcines the lead, and with it the imperfect metals, leaving the gold pure. This process is called _cupellation_, from being performed in a small crucible called a _cupell_. When the gold is mixed with silver, three parts more of silver are put to it, and then the silver is dissolved by nitrous acid, leaving the gold pure. This process is called _quartation_, from the gold being one fourth part of the mass. The fineness of gold is generally estimated by dividing the gold into twenty-four parts, called _carats_. The phrase twenty-three carats fine means that the mass contains twenty-three parts out of twenty-four of pure gold, the remainder being _alloy_, of some baser metal. The fineness of gold may in some measure be discovered by the colour it leaves upon a _touch-stone_, or fine-grained basaltes. Gold is generally found nearly pure, but mixed with earth, or diffused in fine grains through stones. LECTURE XXII. _Of Silver._ Silver is the whitest of all the metals, very ductile, but less so than gold; the thinnest leaves of it being one third thicker than those of gold. It is not calcined in the heat of a common furnace, but partially so by repeated fusion, or a strong burning lens. Sulphureous fumes unite with silver, and tinge it black. The nitrous acid dissolves it, and will hold more than half its weight of it in solution. When fully saturated, this solution deposits crystals, which are called _lunar nitre_, or _nitre of silver_. When these crystals are melted, and the water they contain driven off, a black substance, called _lapis infernalis_, or _lunar caustic_, is formed. This is used as a cautery in surgery. A strong heat will decompose this lunar nitre, and recover the silver. Though the nitrous acid dissolves silver the most readily, the marine acid will deprive the nitrous of it, and form a substance called _luna cornea_, because, when it is melted and cold, it becomes a transparent mass something resembling _horn_. From this luna cornea the purest silver may be obtained. The vitriolic acid will likewise deprive the nitrous of the silver contained in it, and form a white powder, not easily soluble in water.