Many of the ores in their natural state are said to be _mineralized_ with arsenic or sulphur, those substances being intimately united with the metallic earths.
In order to convert the ores into metals, some of them are first reduced to powder, to wash out the earthy or saline particles. They are then kept in a red heat, which the workmen call _roasting_, in order to drive away the arsenic, or sulphur, which are volatile; and in the last place they are fused in contact with charcoal, or other substances containing phlogiston; and to promote the fusion, lime-stone is frequently mixed with them. When the operation is completed, the unmetallic parts are converted into glass, or _scoria_, which lies on the surface, whereas the metal is found at the bottom.
To discover the quantity of metal in a small piece of ore is called _assaying_.
When metals are fused together, the specific gravity, fusibility, and other properties are changed, and in such a manner as could not be discovered from the properties of the constituent parts.
Gold is the heaviest of all metallic bodies except platina. It appears yellow or reddish by reflected light, but green or blue by transmitted light, when it is reduced to thin plates.
Though gold undergoes no change in a common furnace, or burning lens, it may, in part, at least, be calcined by the electric shock.
Gold has the greatest _ductility_, and in wires of equal diameters, it has the greatest _tenacity_, of all the metals. One grain of it may be made to cover 56 square inches; some gold leaf being less than a 200,000th part of an inch thick; and when it is made to cover a silver wire, the gold upon it may not be more than one twelfth part of the thickness of the gold leaf.
This metal is soluble in aqua regia; and being precipitated by a volatile alkali, makes a powder called _aurum fulminans_, which is one fourth heavier than the gold, and explodes with great violence in a heat something greater than that of boiling water.
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.