AMALGAMS. (Fn. AMaigaMe j GER., Amalgam.)—Mercury unites with a larc-,e 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 ia pressed through the pores of a chamois-leather bag a small portion of mercury passes through, leaving behind the solid amalgam, which, on examina tion, 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.
Tho chemical affinity which causes the 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 ht at 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 fine coating of metallic gold, which, on burnishing, regained its beautiful and characteristic lustre.
The following are some of the most important amalgams:— Copper Amalgam.—Thero are several methods of prepsring this amalg,am, 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 half an hour. After this the water is repeatedly changed until it is no longer blue. The mass ia then dried, kneaded well and allowed to harden, when it consists of an amalgam of seven parts mercury with three parts copper. The peculiarity of this amalgam is its property of softening when k-neaded and becoming quite hard again after standiog 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.
Gold Amalgam.—This amalgam is formed when mercury ia heated with powdered gold or gold-foil.
It consists usually of two parts of gold to one of mercury. It has been found native near Mariposa, in California, and in the platinum region of Columbia.
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 ia termed. This is a sitnple iron trough, usually 4 or 5 feet long, 12 or 14 inches wide, and 9 inches deep, cast with a solid bottom 9 or 10 inches in thickness. The ends of the battery are lined with amalg,amated copper plates, while another plate of the same kind, about 10 or 12 inches wide and ss long ss the inside of the battery, is so fixed in a frame that it may be set and secured in an inclined position behind the stamps by which the ore is crushed. A similar .plate, though narrower, is generally used on the front or discharge side of the battery. Water is introduced into each battery by a number of small pipes. The mercury is placed in the batleries in small quantities, and it 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 copper plate, the remainder being caught by the inclined plate placed outside the battery for that purpose. This plate is fixed at such an inclination that the stream passes steadily over its surface and allows the amalgam to adhere to it. The plate is grooved at right angles to the line of motion, thereby affording increased facilities for the contact of the two metals and the amalgamation of the gold. Leaving this plate, the stream flows into tanki or basins, carrying with it small quantities of amalgam not retained by the plate, and a little unamalgamated gold. The amalgam formed in the batteries and on the plate is cleaned up at intervals varying in length according to the richness of the ore. The outside plates are cleaned by carefully 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.