MERCURY AND ITS AMALGAMS Mercury.—Mercury is the only pure metal which is liquid at ordinary tem peratures. It has been known from remote antiquity together with its chlorides, corrosive sublimate and red oxide, which were known to the Ara bians, and calomel, known to the al chemists. Its sulphide cinnabar has been used as a pigment from ancient times. Pure mercury has a pure sil ver-white luster. Like silver, it is tarnished by the fumes of sulphur or dust which adheres to and adulter ates it.
Mercury mixes freely with most of the common metals, especially lead, tin, zinc, and copper. Contact with other metals often ruins mercury or greatly lessens its value. No metal should be allowed to touch it except iron or platinum.
Pure mercury forms in globules, which run freely over glass or other smooth surface. The presence of other liquids causes it to be somewhat sticky and to " drag a tail " behind when flowing over smooth surfaces. Hence this is a test of its purity.
Mercury freezes at a temperature of 39° F. Hence, while used for ther mometers at ordinary temperatures, it is not suitable for those used in arctic ,regions, where it is replaced by alco hol or some other spirit.
Mercury is also used in amalgama tion with zinc for voltaic batteries, for mirrors, barometers, thermometers, steam and other pressure gauges, and with copper in dental amalgams.
Amalgams.—Alloys of mercury with other metals are called amalgams; among these are the dental amalgam with copper, an amalgam with tin for coating mirrors, and amalgams of the precious metals used in mining.
Calomel, a chloride of mercury, is used in medicine, and corrosive sub limate is used as a preservative and insecticide. The mineral cinnabar, a sulphide of mercury, forms the beau tiful pigment known as vermilion.
To Coat Mirrors with Mercury.— Spread on a solid level table a sheet of tin foil the size of the mirror, hav ing a ledge about its edge to prevent the mercury from flowing over at the sides. Pour on a little mercury, and rub it over the surface with a piece of felt, taking care not to wrinkle the foil. Now pour on mercury deep enough so that its upper surface will be liquid. Then push the glass along the layer of mercury with its lower edge below the surface, so as to keep out the air and push along any im purities, and let it rest on the mer cury. Tilt the table to let the excess of mercury run off into a trough pro vided for it. Lay on a weight of the same size and shape as the glass so as to exert a uniform pressure, and let stand until hard and dry. The time
required varies from a few days or weeks to a number of months, depend ing upon conditions that are not fully understood. During this process mer cury must be kept from the fumes of sulphur, sewer gas, burning matches, or anything containing sulphur, as rub ber and the like. After the amalgam dries, the mirror may be lifted care fully and backed with thin boards or otherwise to exclude the air.
Or lay a sheet of tin foil on a mar ble slab, a table, or other smooth sur face, and rub it with mercury, using a hare's foot or a piece of soft cham ois. Use about 3 drams of mercury to 1 square foot of foil. Continue the rubbing until the two metals in corporate and the foil becomes bril liant. Lay the plate of glass upon it and load it with weights in such a way that the pressure will be exactly equal over all parts of the glass. Any excess of mercury will be squeezed out and the tin foil will adhere to the glass.
To Repair Mirrors.—To renew the silvering from the back of a mirror, lay it face downward on a smooth surface and prepare a piece of tin foil large enough to patch the damaged part by rubbing it with mercury. Place the patch in position, lay a sheet of paper over it, and put on it a weight having a perfectly flat sur face and heavy enough to press it down tightly. Let the mirror lie in this position for a day or two and the foil will adhere to the glass.
Or the silvering may be removed from a broken mirror and used to patch a mirror which is scratched or damaged.
Platinum. — A whitish steel-gray metal, very ductile and malleable, and, like gold, not liable to tarnish or cor rode. It is one of the heaviest of the metals, being about 91i times as heavy as water.
Platinum cannot be melted with or dinary heat, hence it is commonly used as a material for crucibles in which other metals are to be melted. It may be dissolved, like gold, in aqua regia, and forms alloys with gold and silver and many other metals which are more fusible than pure platinum. Combined with the rare metal iridium it forms an extremely hard alloy, which is used for gun vents and standard weights and measures.
Platinum wire is used in incandes cent electrical lamps. Its price varies from $9 to $12 an ounce.
To Solder Platinum. — To solder platinum, use pure gold with borax.