Lecture Xxi.

Of Ores.

Metallizable earths, commonly called ores, when united to phlogiston, make the metals, distinguishable for their specific gravity, their opacity, shining appearance, and fusibility.

All the proper metals are malleable, and those which are not so are called semi-metals.

The metals again are subdivided into the perfect and imperfect. The former, which are gold, silver, and platina, suffer no change by fusion, or the longest continued heat: whereas heat calcines or dissipates the phlogiston of the imperfect metals, which are mercury, lead, copper, iron, and tin, so that they return to the state of earth; and this earth is always heavier than the metal, though of less specific gravity, having received an addition of weight from water or air: but these earths, or ores, being exposed to heat in contact with substances containing phlogiston, again become metals, and are then said to be revived.

The semi-metals are bismuth, zinc, nickel, regulus of arsenic, of cobalt, of antimony, of manganese, of wolfram, and of molybdena.

All metallic substances are crystallizable, and each in a peculiar form, which is discovered by leaving a hole in the bottom of the crucible in which they are melted, and drawing out the stopper, when the mass is beginning to lose its fluidity.

Some of the metals will not unite to others when hot, and others of them will; and such as will unite with others are called solders. Thus tin is a solder for lead, and brass, gold, or silver, for iron.

Ores are never found in regular strata, like the different kinds of earth; but in places which have formerly been cavities, running in all directions, with respect to the regular strata, and commonly called veins.

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.

Of Gold.

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.

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.