alloys having certain desired properties, but the use of commany other combinations are employed to produce white In addition to the alloys given in the preceding table

plicated alloys is much to be deprecated, for, as a rule, the stability is much greater the simpler the mixture; and although a useful alloy may be produced by careful manipulation, the proportions are disturbed when the scrap is remelted, or mixed with new metal and remelted. The author would therefore recommend a triple mixture of tin, antimony, and copper in preference to a more complex one, especially for wrought work.

The preparation of Britannia metal is based on the idea of rendering tin harder, tougher, more sonorous, and more easily polished. It furnishes castings as fine and sharp as those made with the most fluid alloys of tin and lead, copper and zinc, etc. It acquires a finer polish than alloys of tin and lead, because the latter is too soft to bear the action of emery and other polishing materials. A little brass is sometimes added, and bismuth increases the fusibility. The following proportions will serve as a guide :—

These are melted together; then the alloy so formed is fused with 15 to 20 per cent more tin, according to the judgment of the operator. A very complex alloy, termed by the inventor English metal, consists of—

Britannia metal is best prepared by melting the copper first, then adding the antimony with a portion of the tin, and finally adding the remainder of the tin. The mixture requires to be vigorously stirred in order to thoroughly incorporate the contents. If the alloy upon fracture shows a nonhomogeneous structure, it requires to be rapidly remelted under a layer of charcoal, to prevent oxidation. Certain

alloys are very elastic, and well fitted for making wire.

In

this respect they possess nearly the same amount of tenacity as pure tin. With good mixtures the alloys may be easily stamped and rolled, although they have a tendency to crack on the edges.

Dr. Karmarsch, who has carefully studied the properties of Britannia metal, states that the specific gravity is diminished by rolling, the metal having a greater density in the cast state than after rolling. He explains this by assuming that the particles under the pressure of the rolls have a tendency to become separated, their softness and tenacity not being great enough to allow of a regular and uniform compression. This is not an isolated fact. M. le Brun has found a lower specific gravity for certain alloys of copper and zinc which have been laminated or hammered.

To obtain clear and fine castings of Britannia metal it is best to use brass moulds. Before casting, the moulds must be heated, and coated on the interior with a mixture of lamp-black and oil of turpentine, or lamp-black alone, in order to prevent the metal adhering to the mould. The moulds can be so coated by smoking them with the flame of a lamp, filled with oil of turpentine. Instead of lamp-black some manufacturers use reddle or red-chalk, mixed to a uniform mass with water.

Many articles cannot be cast in one piece with ordinary moulds; but the parts require to be cast separately, and afterwards soldered together. 1"To cast an article such as a coffee-pot in one piece requires great skill and judgment. The separate parts of the mould having been blacked, are put together, and the whole heated nearly to the temperature of the melting point of Britannia metal. The metal is then poured in until the mould seems entirely filled. After waiting until it may be supposed that a sufficiently thick layer of metal is solidified, the mould is quickly turned over to allow the still liquid portion to run out. The inside of articles obtained by this method is sometimes roughly crystalline. This is due to the metal beginning to crys1 Brannt, Metallic Alloys, p. 280.

tallise, and the corners and edges of the small crystals being exposed by pouring out the liquid portion of the metal. The interior of the articles can be smoothed by a burnisher while they are still in the mould."

FUSIBLE ALLOYS

This name is given to a series of alloys which melt at comparatively low temperatures, and consist chiefly of tin, lead, and bismuth. Before considering these triple alloys a brief account will be given of the combinations of tin and bismuth alone.

§ 102. Tin and Bismuth.—Both these metals possess low melting points, and they readily combine in all proportions when fused together. A very small proportion of bismuth imparts to tin more hardness, sonorousness, lustre, and fusibility. On that account a little bismuth is added to tin to increase its hardness. However, bismuth being easily oxidised, and often containing arsenic, it is not advisable to use much of that metal in alloy with tin for culinary vessels, etc. The alloys of tin and bismuth are more fusible than either of the metals taken separately.

1"An alloy of 354 parts (3 atoms) of tin and 420 parts (2 atoms) bismuth, when cooled from a state of fusion, exhibits but one solidifying point; inasmuch as it first cools regularly down to 143°, and then remains at that temperature for some time, till the latent heat, set free in the solidification of the alloy, has had time to escape. But all other alloys of these metals likewise exhibit a higher solidifying point, the excess of one or the other metal, or rather, another definite alloy containing an excess of one of the two metals solidifying first, and afterwards, at 143° C., the hitherto fluid alloy containing Sn,Bi2. The higher solidifying point, or point of separation, is 190° for Sn Bi, 160 for Sn,Bi, 170 for Sn,Bi„, and 190° for SnBi2.”

3

1 Rudberg. Pogg. Ann. tom. xviii. p. 240.

The

percentage composition of the above alloys with their melting points is shown in the following table :—

Chaudet states that an alloy of 40 parts tin to 1 part bismuth is perfectly ductile; the addition of 1 part lead diminishes its extensibility. An alloy of 25 parts tin to 1 part bismuth is slightly ductile. According to Lewis an alloy of 8 parts tin to 1 part bismuth melts at 199° C. An alloy of 3 parts tin to 1 part bismuth is pulverisable, of a dull-gray fracture, and has a specific gravity of 7.776. An alloy of 2 parts tin to 1 part bismuth melts at 166° C. An alloy of 1 part tin to 1 part bismuth is perfectly brittle, pulverisable, with a fine-grained fracture, and has a specific gravity of 8.345. It melts at 138° C. according to Lewis, and expands considerably in solidifying. Döbereiner found that the alloy Sn,Bi, melts between 131° and 137° C.

§ 103. Alloys of Tin, Bismuth, and Lead.-Alloys containing these three metals are more fusible than those containing only two of them. They are useful for ascertaining a given temperature; for making easily melted plastic metals, in order to obtain casts of delicate objects which may be damaged by too high a temperature; for making very fusible soft solders; and lastly, as a matter of precaution, for such apparatus as is liable to be instantaneously destroyed by a sudden and excessive increase of temperature. In this connection may be mentioned the fusible safety plugs of boilers, which were formerly very extensively used.