The alloys numbered 7 and 8 are of especial interest. The first AgCu2 is Levol's homogeneous alloy, which remains uniform in composition, while many others undergo liquation on slow cooling. The alloy AgCu has the simplest atomic relation and the lowest melting point.

In studying the phenomena of liquation, the alloys were cast in moulds of fire-brick, in which the metal (about 50 ounces) could be quickly or slowly and uniformly cooled. The results showed that the homogeneity of Levol's alloy is slightly disturbed by this method of casting; and, on the other hand, that alloys which contain more than 71.89 per cent of silver hardly show signs of rearrangement when the solidification takes place gradually. Two alloys containing 63 and 33.3 per cent of silver respectively, were found to be far from homogeneous, and in the former the arrangement was influenced by gravity, the base of the casting being richer in silver. The following table contains a summary of Roberts-Austen's results :

In Ia and II a the centre of the cubes was the richest and the corners the poorest.

In Ib and IIb the slight variations followed the same law.

In III the corners were generally richer than the centre. In IV it was supposed that gravity had influenced the alloy, the lower parts being richer than the upper.

In V the variations do not follow any known law. With regard to standard silver cast under ordinary conditions, the tendency of the copper and silver to separate appears to depend upon the inequality of the rate of cooling in the different parts of the ingot. The act of cooling causes a partial separation of the copper at the parts first cooled, and those parts which solidify last are generally richer in silver.

Silver-copper alloys are subject to change when strongly heated in air, the copper being oxidised. If the alloy contains much copper, the silver will also, to some extent, be oxidised, but in a less degree than the copper. When such alloys are just heated to redness in an ordinary muffle with accession of air the discoloration of the surface is proportionate to the amount of copper. The following table is due to Chaudet 1:-

1 L'Art de l'Essayeur. Par M. Chaudet, pp. 77, 78.

§ 159. Standard or Sterling Silver.-British silver coin and plate contain definite proportions of silver and copper, regulated by law. Silver coin contains 11 oz. 2 dwts. of silver per lb. troy, or 925 parts of silver per 1000, the remaining 75 parts being copper. It should be mentioned that the copper, added to the silver, is termed the alloy, whereas, strictly speaking, the whole body, containing silver and copper, is the alloy. Articles of silver to be Hall-marked must be made of standard silver, as no other quality is allowed by law to be assay-marked.

In preparing standard silver for Hall-marking it is necessary to add a little more than the above stated amount, because commercial fine silver is never perfectly pure; so that instead of using 18 dwts. 12 grs. per ounce, use 18 dwts. 14 grs. The quality commonly used by silversmiths is generally below the standard, when not Hall-marked, and contains 18 dwts. of silver per ounce, or 900 parts per 1000.

§ 160. The various alloys of silver and copper employed for manufacturing purposes are represented in the following table 1:

No. VIII is about the commonest alloy it is possible to make without a perceptible yellow cast being imparted to the colour. A commoner variety may, however, be made by adding a third metal such as nickel, which will be subsequently referred to.

The alloys for coin and plate until recently legal in France were as follows 1

:

The 10 centimes pieces, termed "billon," have not been made since 1810. The standard for all French silver coins under the 5-franc piece has been lowered to 835 of silver per 1000 of alloy.

In Germany there are four silver standards, which are as follows:

1 L'Art de l'Essayeur. Par M. Chaudet, p. 335.

No other metal than copper is allowed to be alloyed with the silver.

Their

Elliot and Storer 1 have detected the presence of lead in silver coins of the United States and other countries. results are given in the following table :—

The lead in American coins is accounted for by the zinc, used in the reduction of the chloride of silver, containing some lead. The silver at the French Mint is precipitated by copper from an acid solution of sulphate of silver, and the acid probably contains sulphate of lead. The lead in English coin may be due to imperfect cupellation.

The following table gives the composition of coins of various countries:

1 Amer. Acad. of Arts and Sc. 1860, p. 52.