AMALGAM, an alloy in which mercury is an important constituent. Silver and gold amalgams occur in nature to a limited extent, but most of the amalgams are of artificial origin. Four general methods of forming them may be noted: (1) By direct contact of mer cury with the metal to be amalgamated. Amal gams of antimony, arsenic, bismuth, cadmium, gold, lead, magnesium, potassium, silver, sodium, tellurium, thorium, tin and zinc may be obtained in this way. The different elements mentioned combine with the mercury with va rying manifestations of affinity, the amalgama tion of sodium being attended with the pro duction of heat and light, while in the case of zinc it is often necessary to bring the zinc and mercury together in the presence of dilute acid before they will combine evenly and smoothly. (2) By immersing the metal to be amalgamated in a solution of a salt of mercury. Copper, gold, platinum and silver can be amalgamated in this way. (3) By reversing the process last described and bringing mercury in contact with a salt of the metal whose amalgam is desired. The mercury, in certain cases, will partially re place the metal in solution, the portion so re placed combining with the mercury with the production of the desired amalgam. A val uable modification of this method consists in substituting for the metallic mercury an amal gam of zinc or of sodium, the zinc or sodium changing places with the metal in solution. Amalgams of bismuth, calcium, chromium, iridium, iron, magnesium, manganese, osmium, palladium and strontium may be prepared by the use of sodium amalgam. (4) By electrol ysis, the metal whose amalgam is desired be ing used as the cathode in a solution of a mer curial salt. (The cathode may also be metallic mercury, and the electrolyte a salt of the metal whose amalgam is desired). This process is in commercial use for the production of sodium hydrate, a solution of sodium chloride (com mon salt) being electrolyzed with a mercury cathode. The cathode absorbs the sodium with the formation of sodium amalgam, which is subsequently decomposed by contact with water. In practice the process is continuous, a part of the mercury cathode being exposed to the electrolytic bath, while another part is si multaneously exposed to the action of the water.
In the formation of amalgams there is usu ally but little thermal effect. In the case of sodium and potassium, however, a very con siderable amount of heat is evolved; and in the format;on of amalgams of bismuth, lead and tin, heat is absorbed.
There is considerable evidence in favor of the view that many amalgams contain definite compounds of mercury and the other con stituent metals. Thus when certain amalgams are heated (say) to the boiling point of sul phur, the excess of mercury present appears to be volatilized, so as to leave a body behind that has a definite chemical composition. In this
way Sonza obtained amalgams having the ap parent composition Ana-1g, Ag.2Hg, Cu,.Hg, Naslig, and K,Hg, the last-mentioned being silvery in appearance and crystalline in struc ture. But it is said that all these amalgams, as well as many others, continue to lose mercury slowly when the temperature is maintained high; and this fact, while not disproving the existence of a definite compound of mercury and the metal, lessens its probability. Amal gams having the composition CuHg, AgHg, FeHg, Zn,Hg, Pb,Hg and PtHg, have also been prepared by expelling the excess of mer cury from amalgams richer in that metal by exposure to a pressure of 70 tons to the square i inch. One of the most interesting amalgams from the standpoint of chemical theory is the amalgam of the hypothetical radical °ammo nium," which is described under AMMONIA.
The affinity of mercury for gold is put to practical use in mining for the recovery of small particles of gold from auriferous gravel or crushed quartz. The details of the process vary somewhat according to the nature of the material from which the gold is to be ex tracted; but in general it may be said that the pulverized gold-bearing quartz or gravel is washed, in a finely divided state, over a plate of amalgamated copper, to which the gold par ticles adhere. From time to time the gold amalgam is scraped from the copper plate and more mercury is added. The presence of sul phur (from pyrites) seriously interferes with this process, by causing the formation of a sulphide of mercury which destroys the effi ciency of the amalgamated plate. The mercury is then said, in miners' parlance, to become °sick?' To prevent this the ore, if originally rich in pyrites, is roasted to expel the sulphur before being submitted to amalgamation. See GOLD.
Mirrors are silvered by amalgams. One of the simplest of those so used is composed of one part of tin to three of mercury. A supe rior amalgam for this purpose contains two parts of bismuth, one part each of lead and tin and four parts of mercury. In dentistry the °silver filling* used for closing the cavities in teeth is an amalgam. Its composition varies somewhat, but a preparation containing two parts of mercury and one part of pulverized zinc gives excellent results. It hardens quickly and expands slightly in solidifying, thus filling the cavity tightly. Amalgams of copper, silver, lead and tin have a volume smaller than the sum of the volumes of their constituents. See BATTERY.