Mercury unites with a large number of metals, forming definite chemical compounds called "amalgams." Some of these are solid, while others exist in a fluid state. It is probable, however, that fluid amalgams merely represent a solution in excess of mercury of some fixed compound of mercury with another metal, inasmuch as when a quantity of such fluid amalgam is pressed through the pores of a chamois-leather bag, a small portion of mercury passes through, leaving behind the solid amalgam, which, on examination, is generally found to have a fixed chemical constitution. The fluidity of an amalgam seems therefore to depend upon the presence of an excess of mercury over and above the amount theoretically required to enter into combination with the other metal.
The chemical affinity which causes mercury to combine with other metals is generally of a feeble character. Gentle pressure will drive out a considerable quantity of the combined mercury, leaving a combination in altogether different proportions from the original one. A moderate heat also is sufficient to decompose almost any amalgam. This fact was formerly made use of in the process known as water-gilding. The article to be gilded was covered with an amalgam of gold with excess of mercury, and then subjected to a strong heat. The mercury was driven off, leaving the article covered with a tine coating of metallic gold, which, on burnishing, regained its lustre.
The following are some of the most important amalgams:-
There are several methods of preparing this, the following being, perhaps, the best:-A mixture of finely-divided metallic copper (obtained by precipitating copper sulphate with metallic iron) and mercurous sulphate is triturated under hot water for 1/2 hour. After this, the water is repeatedly changed until it is no longer blue. 1 he mass is then dried, kneaded well, and allowed to harden, when it consists of an amalgam of 7 parts mercury with 3 of copper. The peculiarity of this amalgam is its property of softening when kneaded, and becoming quite hard again after standing some hours. It has been used by Parisian dentists as a stopping for decayed teeth, though, owing to the poisonous nature of the copper, it is not to be recommended for this purpose.
This is formed when mercury is heated with powdered gold or gold-foil. It consists usually of 2 parts of gold to 1 of mercury. It has been found native near Mariposa, in California, and in the platinum region of Colombia.
The readiness with which mercury combines with gold is made use of in the extraction of the latter from its ores. The ore is crushed in an iron mortar, or battery, as it is termed. Water is introduced into each battery by a number of pipes. Mercury is placed in the batteries in small quantities, and unites with the gold as the latter is liberated by the crushing process. The larger portion of the amalgam is afterwards found in the batteries, adhering to the plates, the remainder being caught by inclined plates placed outside the battery. The plates are cleaned by scraping off the adhering amalgam, first gently with a knife, and finally with a thick piece of hard gum or rubber, which scrapes the surface closely without cutting or scratching it. The plates are then washed with water, and prepared for use again by sprinkling mercury over them, and spreading the same evenly by means of a cloth, thus forming a freshly amalgamated surface.
Iron will not unite with mercury under ordinary conditions. Small quantities of an iron amalgam have, however, been formed by immersing sodium amalgam (containing 1 per cent. sodium) in a clear, saturated solution of ferrous sulphate.
FRONTISPIECE. COLOUE SCALE FOR TEMPERING Iron. (Sec p. 268.)
This compound is formed by the union of mercury with finely-divided silver. Native silver amalgam has been found at Moschel-landsberg, in the Palatinate, and in several other places. Mercury is used for silver extracting, in a process somewhat similar to that described above for the extraction of gold.
Sodium and mercury combine readily under ordinary .conditions by being brought into contact one with another. The union is attended with much hissing and spluttering, and with a considerable evolution of heat.
Tin and mercury combine readily at ordinary temperatures. If 3 parts mercury be brought into contact with 1 of tin, 6-sided crystals of tin amalgam are formed. Tin amalgam is used for silvering looking-glasses. When pulverized and rubbed on the polishing-stone, it forms a kind of mosaic silver. Electric amalgam may be made by melting tin and sine together in various proportions in a porcelain crucible. The mixture is well stirred up, and when on the point of solidifying, the mercury is added and worked into the mass. The whole is next transferred to a mortar warm enough to keep the amalgam soft while it is well worked together, after which' a piece of tallow or lard, not quite equal in bulk to the mass, is kneaded in until the amalgam attains the proper consistency.
Zinc Amalgam is formed by mixing and triturating zinc filings with mercury, at a heat somewhat below the boiling-point of the latter. It is usually prepared by pouring mercury into zinc at the temperature at which the latter is just kept in a fused state. Care must be taken to keep the liquid 6tirred, and to add the mercury slowly and in as fine a stream as possible.