the dynamo-electric machines of Gramme, Siemens, and others have rendered it possible to cheaply convert the mechanical movement of the steam engine into an electric current. One must think that the application of the electric current, which has long since given such important results in chemistry, will, in the near future, play an important part in technical processes, the example being shown by electric lighting.11

The alloys of copper with certain metals, and especially with zinc and tin, are easily formed by directly melting the metals together. They are easily cast into moulds, forged, and worked like copper, whilst they are much more durable in the air, and are therefore frequently used in the arts. Even the ancients exclusively used alloys of copper, not pure copper but its alloys with tin or different kinds of bronze (Chapter XVIII.). The alloys of copper with zinc are called brass or 'yellow metal.' Brass contains about 32 p.c. of zinc ; generally, however, it does not contain more than 65 p.c. of copper. The remainder is composed of lead and tin, which usually occur, although in small quantities, in brass. Yellow metal contains about 40 p.c. of zinc. The addition of zinc to copper changes its colour to a considerable degree; with a certain amount of zinc the colour of the copper becomes yellow, and with a still larger proportion of zinc an alloy is formed which has a greenish tint. In those alloys of zinc and copper which contain a greater amount of zinc than of copper, the yellow colour disappears and is replaced by a greyish colour. But when the amount of zinc is diminished to about 15 p.c., the alloy is red and hard, and is called 'tombac.' A contraction takes place in alloying copper with zinc, so that the volume of the alloy is less than that of either metal individually. The zinc volatilises on prolonged heating at a high temperature, and metallic copper remains behind. When heated in the air, the zinc oxidises before the copper, so that all the zinc alloyed with copper may be removed from the copper by this means. An important property of brass containing about 30 p.c. of zinc is that it is soft and malleable in the cold, but becomes somewhat brittle when heated. We may also mention that ordinary copper money contains,

11 Commercial blue vitriol generally contains ferrous sulphate. The salt is purified by converting the ferrous salt into a ferric salt by heating the solution with chlorine or nitric acid. The solution is then evaporated to dryness, and the unchanged cupric sulphate extracted from the residue, which will contain the larger portion of the ferrie oxide. The remainder will be separated if cupric hydroxide is added to the solution and boiled; the cupric oxide, CuO, then precipitates the ferric oxide, FeO3, just as it is itself precipitated by silver oxide. But the solution will contain a small proportion of a basic salt of copper, and therefore sulphuric acid must be added to the filtered solution, and the salt then crystallised. Acid salts are not formed, and cupric sulphate itself has an acid reaction on litmus paper.

VOL. II.

C C

for the sake of rendering it hard, tin, zinc, and iron (Cu = 59 p.c.); that it is now customary to add a small amount of phosphorus to copper and bronze, for the same purpose; that copper is added to silver and gold in coining, &c., also to render it hard; that in Germany, Switzerland, and Belgium, and other countries, a silver-white alloy (melchior, German silver, &c.), for base coinage and other purposes, is prepared from brass and nickel (from 10 to 20 p.c. of nickel; 20 to 30 p.c. zine; 50 to 70 p.c. copper), or directly from copper and nickel, or, more rarely, from an alloy containing silver, nickel, and copper. 116

Copper, in its cuprous compounds, is so analogous to silver, that were there no cupric compounds, or if silver gave stable compounds of the higher oxide, AgO, the resemblance would be as close as that between chlorine and bromine or zinc and cadmium; but silver compounds corresponding to AgO are quite unknown. Although silver peroxide-which was regarded as AgO, but which Berthelot (1880) recognised as the trioxide Ag203-is known, still it does not form any true salts, and consequently cannot be placed along with cupric oxide. In distinction to copper, silver as a metal does not oxidise under the influence of heat; and its oxides, Ag2O and Ag,O,, easily lose oxygen.12 Silver does not oxidise in air at the ordinary pressure, and is therefore classed among the so-called noble metals. It has a white colour, which is much purer than that of any other known metal, especially when it is chemically pure. In practice silver is used alloyed, because chemically-pure silver is so soft that it is exceedingly easily worn, whilst, when fused with a small amount of copper, it becomes very hard, without losing its colour. 13

115 Ball (also Kamensky), 1888, by investigating the electrical conductivity of the alloys of antimony and copper with lead, came to the conclusion that there only exist two definite compounds of antimony and copper, whilst the other alloys are either alloys of these two together or with antimony or with copper. These compounds are Cu Sb and Cu4Sb-one corresponds with the maximum, and the other with the minimum, electrical resistance. In general, the resistance offered to an electrical current forms one of the methods by which the composition of definite alloys (for example, Pb,Zn7) is often established, whilst the electromotive force of alloys affords (Laurie, 1888) a still more accurate method. Thus the alloys ZnCu and CusSn were shown to be such that the electromotive force varied distinctly on their formation.

12 Cupric oxide is also able to dissociate when heated. Debray and Joannis showed that it then evolves oxygen, whose maximum tension is constant at a given temperature if fusion does not ensue (cupric oxide dissolves in fused cuprous oxide), and this loss of oxygen results in the formation of cuprous oxide, and on cooling the oxygen is all reabsorbed and cupric oxide again formed.

13 There are not many soft metals; lead, tin, copper, silver, iron, and gold are somewhat soft, and potassium and sodium very soft. The metals of the alkaline earths are already sonorous and hard, and many other metals are even brittle, especially bismuth and antimony. But the very slight significance which these properties have in

Silver occurs in nature, both in a native state and in certain compounds. Native silver, however, is of rather rare occurrence.

A far

determining the fundamental chemical properties of substances (although, however, of immense importance in the practical applications of metals) is seen from the example shown by zinc, which is hard at the ordinary temperature, soft at 100°, and brittle at 200°.

As the value of silver depends exclusively on its purity, and as there is no possibility of telling the amount of impurities alloyed with it from its external appearance, it is customary in most countries to mark an article with the amount of pure silver it contains after an accurately-made analysis known as the assay of the silver. In France the assay of silver shows the amount of pure silver in 100 parts by weight; in Russia the amount of pure silver in 96 parts-that is, the assay shows the number of zolotniks (4-26 grams) of pure silver in one pound (410 grams) of alloyed silver. Russian silver is generally 84 assay-that is, contains 84 parts by weight of pure silver and 12 parts of copper and other metals. French money contains 90 p.c. (in the Russian system this will be 86 4 assay) by weight of silver [English coins and jewellery contain 92.5 p.c. of silver]; the silver rouble is of 83} assay-that is, it contains 868 p.c. of silver-and the smaller Russian silver coinage is of 48 assay, and therefore contains 50 p.c. of silver. Silver ornaments and articles are usually made in Russia of 84 and 72 assay silver. As the alloys of silver and copper, especially after being subjected to the action of heat, are not so white as pure silver, they generally undergo a process known as 'blanching' (or 'pickling ') after being worked up. This consists in removing the copper from the surface of the article by subjecting it to a dark-red heat and then immersing it in dilute acid. During the calcination the copper on the surface is oxidised, whilst the silver remains unchanged; the dilute acid then dissolves the copper oxides formed, and pure silver is left on the surface. The surface is dull after this treatment, owing to the removal of a portion of the metal by the acid. After being polished the article acquires the desired lustre and colour, so as to be indistinguishable from a pure silver object. In order to test a silver article, a portion of its mass must be taken, not from the surface, but to a certain depth. The methods of assay used in practice are very varied. The commonest and most often used is that known as cupellation. It is based on the difference in the oxidisability of copper, lead, and silver. The cupel is a porous cup with thick sides, made by compressing

FIG. 95.-Cupel for silver assaying.

FIG. 96.-Clay muffle.

bone ash. The porous mass of bone ash absorbs the fused oxides, especially the lead oxide, which is easily fusible, but it does not absorb the unoxidised metal. The latter collects into a globule under the action of a strong heat in the cupel, and on cooling solidifies into a button, which may then be weighed. Several cupels are placed in a muffle. A muffle is a semi-cylindrical clay vessel, shown in the accompanying drawing. The sides of the muffle are pierced with several orifices, which allow the access of air into it. The muffle is placed in a furnace, where it is strongly heated. Under the action of the air entering into the muffle the copper of the silver allov is oxidised, but as the oxide of copper is infusible, or, more strictly speaking, difficultly fusible, a certain quantity of lead is added to the alloy; the lead is also oxidised by the air at the high temperature of the muffle, and gives the very fusible lead oxide. The copper oxide then fuses with the lead oxide,

greater quantity of silver occurs in combination with sulphur, and especially in the form of silver sulphide, Ag2S, with lead sulphide or copper sulphide, or the ores of various other metals. The largest amount of silver is extracted from the lead in which it occurs. If this lead be calcined in the presence of air, it oxidises, and the resultant lead oxide, PbO (litharge' or 'silberglätte,' as it is called), melts into a mobile liquid, which is easily removed. The silver remains in an unoxidised metallic state.14 This process is called cupellation.

and is absorbed by the cupel, whilst the silver remains as a bright white globule. If the weight of the alloy taken and of the silver left on the cupel be determined, it is possible to calculate the composition of the alloy. Hence the essence of cupellation consists in

the separation of the oxidisable metals from silver, which does not oxidise under the action of heat. A more accurate method, based on the precipitation of silver from its solutions in the form of silver chloride, is described in detail in works upon analytical chemistry.

14 In America, whence the largest amount of silver is now obtained, they work ores containing not more than p.c. of silver, whilst atp.c. its extraction is very profitable. Moreover, the extraction of silver from ores containing not more than 0.01 p.c. of this metal is sometimes profitable. The majority of the lead smelted from galena contains silver, which is extracted from it. Thus near Arras, in France, they work an ore which contains about 65 parts of lead and 0.088 parts of silver in 100 parts of ore, which corresponds with 136 parts of silver in 100000 parts of lead. At Freiberg, in Saxony, they

Commercial silver generally contains appper, and, more rarely, also other metallic impurities. Chemically pure silver is obtained either by cupellation or by subjecting ordinary silver to the following treatment. The silver is first dissolved in nitric acid, which converts it and the

work an ore (enriched by mechanical dressing) containing about 09 of silver, 160 of lead, and 2 of copper in 10000 parts. In every case the lead is first extracted in the manner described in Chapter XVIII., and this lead will hold all the silver. Not unfrequently other ores of silver are mixed with lead ores, in order to obtain an argentiferous lead as the product. The extraction of small quantities of silver from lead is facilitated by the fact (Pattinson's process) that the molten argentiferous lead in cooling first deposits crystals of pure lead, which fall to the bottom of the cooling vessel, whilst the proportion of silver in the unsolidified mass increases according to the removal of the crystals of lead. The lead is enriched in this manner up to a contents of part of silver, and is then subjected to cupellation on a larger scale.

The ores of silver which contain a larger amount of it are silver glance, Ag2S (sp. gr. 72); argentiferous-copper glance, CuAgS; horn silver or chloride of silver, AgCl; argentiferous grey copper ore; polybasite, M.RS (where M=Ag, Cu, and R= Sb, As), and argentiferous gold. The latter is the usual form in which gold is found in alluvial deposits and ores. The crystals of gold from the Berezoffsky mines in the Urals contain 90 to 95 of gold and 5 to 9 of silver, and the Altai gold contains 50 to 65 of gold and 36 to 38 of silver. The proportion of silver in native gold varies between these limits in other localities. Silver ores, which generally occur in veins, usually contain native silver and various sulphur compounds. The most famous mines in Europe are in Saxony (Freiberg), which has a yearly output of up to 26 tons of silver, Hungary, and Bohemia (41 tons). In Russia, silver is extracted in the Altai and at Nerchinsk (17 tons). The richest silver mines known are in America, especially in Chili (up to 70 tons), Mexico (200 tons), and more particularly in the western states of North America. The richness of these mines may be judged from the fact that one mine in the State of Nevada (Comstock, near Washoe and the cities of Gold Hill and Virginia), which was discovered in 1859, gave an output of 400 tons in 1866. The modes of extracting silver from lead and argentiferous ores are mainly of two kinds-cupellation and chlorination. The first method is applied to the extraction of silver from argentiferous lead. The mode of cupellation on a large scale does not differ from the assay on a small scale (Note 13); it is based on the property of silver of not oxidising in air when heated, whilst the lead and other metals present are oxidised and give fusible oxides, which are easily removed from the silver. In general, the object in the extraction of silver is to convert it into an alloy with lead, which is easily freed from the silver by cupellation. The method of chlorination consists in converting the silver in an ore into silver chloride. This is either done by a wet or a dry method. When the silver chloride is formed, the extraction of the metal is also done by two methods. The first consists in the silver chloride being reduced to metal by means of iron, and, mercury being added to the mass, the mercury dissolves the silver, but does not act on the other metals. The mercury holding the silver in solution is distilled, when the silver remains behind. This method is called amalgamation. The other method is less used, and consists in dissolving the silver chloride in sodium chloride or in sodium thiosulphate, and then precipitating the silver from the solution--by means of iron, for example. In America the chlorination is carried on simultaneously with the amalgamation, but we shall not describe this method, all the more as another process is usually employed in Europe, in which the two processes are carried on separately; the chlorination being done by roasting the ore containing silver with common salt. The salt in volatilising acts on the compounds of silver, and converts them into silver chloride. The amalgamation is then carried on in rotating barrels containing the roasted ore mixed with water, iron, and mercury. The iron reduces the silver chloride by taking up the chlorine from it. The technical details of these processes are described in works on metallurgy.