Among the eutectic alloys, some are known as "fusible alloys." "Wood's alloy" consists of two parts of tin, two of lead, seven or eight of bismuth, and one or two of cadmium; it melts at 66°-7i°; an alloy melting at 6o° (Lipowitz's) consists of tin four parts, lead eight parts, bismuth fifteen parts, and cadmium three parts.
Among the few alloys of definite composition are: ZnPt, Zn3Hg, Cd2TJ, Al3Mn, Sn4Pt, Cu3Sn, and PtHg2. Attempts have been made to separate the constituents of alloys by passing a current of high potential through the melted alloy, with the expectation that electrolysis would take place; but no sign of such separation could be detected; the alloy conducts as a whole; it is probable that the electrons can pass so easily through metals that no combination of a metallic ion with an electron is permanently decomposed.
The following alloys, among others, find practical use:- Sodium amalgam, made by adding small pieces of sodium to mercury, warmed under a layer of heavy paraffin oil; it is liquid when it contains under 1.5 per cent, of sodium, and solid if it contains more. It is used as a source of nascent hydrogen, for it is slowly attacked by water, and more rapidly by dilute acids. On adding this alloy to a concentrated solution of ammonium chloride, a very remarkable phenomenon takes place; the amalgam swells up enormously while retaining its metallic appearance; the product is soft and of buttery consistency; it may consist of ammonium amalgam, and may contain the complex group NH4, or, more probably, (NH4)2. On standing, it rapidly decomposes into mercury, ammonia, and hydrogen.
The addition of a little magnesium to nickel lowers its melting-point considerably, and renders it ductile and malleable. A similar addition of a little aluminium to iron also improves the qualities of the iron. The product is called " Mitis steel."
" Galvanised iron " is produced by passing clean sheets of iron through molten zinc. Alloying takes place on the surface of the iron. Such plates, in a corrugated form, are largely used for roofing buildings. As zinc is a more electropositive metal than iron, the iron is thereby protected from rusting. Iron is similarly coated with tin; but in this case, the iron, if exposed, is prone to rust, for iron loses its electrons more readily than tin, and is attacked by carbonic acid in water more readily than the tin. Indeed, rusting proceeds in an accelerated rate owing to the presence of the tin, for the two metals form a couple.
To deprive commercial lead of the silver which it almost always contains, zinc is stirred up with the molten metal; the zinc dissolves the silver and floats to the surface of the lead; it is allowed to harden, and the cake is then removed. The silver and zinc are separated by distillation of the more volatile zinc; the lead is freed from zinc by melting it in an oxidising atmosphere, when the more easily oxidisable zinc is first oxidised, and can be removed as dross. This is Parke's process for desilverising lead.
The alloy of zinc with copper is termed brass, pinchbeck, Muntz metal, and tombac. English brass usually contains 70 per cent, of copper and 30 of zinc. It is made by melting the copper and adding the molten zinc. The addition of nickel (Cu 52 per cent., Zn 23 per cent., Ni 15 per cent.) yields "German silver," of which spoons, forks, and coins are made. Electroplate has usually a basis of this alloy, and is covered with silver by depositing it from its double cyanide with potassium. Zinc coated over with a superficial layer of zinc amalgam is not attacked by dilute sulphuric acid, and is therefore used as the negative pole of most batteries; it is only on connecting with some less electropositive metal that hydrogen is evolved from the latter, while the zinc dissolves.
" Aluminium bronze " is an alloy of aluminium with copper, containing from 2 to 11 per cent, of the former metal. It resembles gold in colour, and is employed in the manufacture of imitation jewellery.
" Ferrochrome " and " ferromanganese " are produced by simultaneous reduction of ores of iron and chromium, or of iron and manganese. Their addition in small quantity to iron improves its quality. Iron containing about 1 o per cent, of manganese is known as " spiegel iron," for it crystallises in large brilliant plates. Tungsten, too, is sometimes added to iron to increase its hardness.
" Pewter" is an alloy of 80 per cent, of lead with 20 per cent, of tin; plumbers' solder consists of two parts of lead to one of tin; " Britannia metal" is made of equal parts of brass, tin, antimony, and bismuth.
" Bronze " is one of the most ancient alloys, and used to be made by reducing together copper ores and tin ores. It often contains twenty-two parts of tin and seventy-eight parts of copper. Its hardness is greatly increased by the presence of a trace of phosphorus. " Speculum metal," for astronomical mirrors, is made by alloying thirty-two parts of tin with sixty-seven of copper and one of arsenic. It takes a very high polish. Copper is easily tinned by melting the tin in the vessel, and pouring out the excess; this is frequently done to vessels required for cooking.
" Type-metal" is an alloy of lead and antimony, containing 18 per cent of the latter. It expands slightly on solidifying, and consequently when cast in the mould it takes an accurate impression and forms a clean-cut type.
The "Pattmson's process" is a rival of the Parke's process in desilverising crude lead. The lead is melted and allowed to solidify partially; the solid portion consists of nearly pure lead. The liquid portion contains the silver. By repetition of the process, the lead may be nearly completely deprived of silver; and an alloy rich in silver may be obtained, from which the lead may be removed by cupellation. ,
Osmiridium is a native alloy of osmium and iridium; it is extremely hard, and it is used for pointing gold pens and for the bearings of small wheels. An alloy of platinum with 10 per cent, of iridium is the metal employed for crucibles.
An alloy of copper and silver is used for coinage; English coins contain 7.5 per cent, of copper. The alloy must be rapidly cooled, else it ceases to be homogeneous. Gold is also alloyed with copper for coinage; pure gold is a soft metal. The English standard is eleven parts of gold to one of copper; in France and the United States, nine of gold to one of copper. The richness of such an alloy is measured in " carats." Pure gold is " 24-carat gold ;" u 18-carat gold" contains eighteen parts of gold and six of copper.
The study of the chemistry of metallic alloys was for long neglected, but of recent years much has been done. It is curious to think that the successful solution of many chemical problems is to be expected from careful examination of this class of substances, which was the first to engage the attention of the chemists of the remote past.