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 crysBrannt, 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,Bi. The higher solidifying point, or point of separation, is 190° for Sn Bi, 160 for Sn,Bi, 170 for Sn,Big, and 190° for SnBi,." The 1 Rudberg. Pogg. Ann. tom. xviii. p. 240.

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

An

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. alloy of 2 parts tin to 1 part bismuth melts at 166° C. 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.

An

§ 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.

The fusible alloys are based on the property of certain metals to become more fusible when combined than they were when taken singly. Bismuth, tin, and lead especially follow this rule. When cadmium is added in addition, the alloys are still more fusible. It is difficult to obtain these alloys in a perfectly homogeneous state, as they have a tendency to become decomposed while solidifying from a state of fusion, the lead separating somewhat to the bottom of the mass. The most fusible alloy of lead, bismuth, and tin consists of equal parts of lead and tin, and twice the quantity of bismuth; it melts at 94° C. According to Erman, it dilates in an anomalous manner when heated. It expands regularly from 32° to 95° C. and then contracts gradually to 131°; at which point it occupies a less bulk than it did at 32°; it then expands till it reaches 174°, and from that point its expansion is uniform. On account of this property of expanding as it cools, while still in the soft state, it is much used for taking impressions from dies, as even the faintest lines are reproduced with the minutest accuracy. A peculiarity of this alloy is that it will become hot again, enough to burn the fingers, after it has been cooled in cold water. The cause of this phenomenon is attributed to the effect of latent heat. During the solidification and crystallisation of the inside portions of the alloy, the latent heat of these parts is immediately transmitted to the cooled surface.

Mr. Darcet has studied the properties of the following alloys

1. Bismuth 70, lead 20, tin 40.-Softens at 100° C. without melting, and may be kneaded between the protected fingers.

2. Bismuth 80, lead 20, tin 60.-Softens at 100° C. and is easily oxidised. There is, however, too much tin.

3. Bismuth 80, lead 20, tin 40. 4. Bismuth 160, lead 40, tin 70. 5. Bismuth 90, lead 20, tin 40.

These three alloys become more or less soft at 100° C. No. 4 becomes softer than Nos. 3 or 5.

6. Bismuth 160, lead 50, tin 70.-Becomes nearly fluid at 100° C.

7. Bismuth 80, lead 30, tin 40.-Becomes liquid at 100° C., but not very fluid.

8. Bismuth 80, lead 40, tin 40.-Becomes very liquid at 100° C.

9. Bismuth 80, lead 70, tin 10.-Becomes soft at 100° C., but does not melt.

10. Bismuth 160, lead 150, tin 10.-Neither liquid nor soft at 100° C.

These alloys are generally harsh; nevertheless they may be cut. Their fracture is a dead blackish-gray. They are rapidly tarnished in the air, and more so in boiling water, in which they become covered with a wrinkled pellicle, which falls as a black powder. Messrs. Parkes and Martin have also studied the properties of fusible alloys of tin, bismuth, and lead, and recommend these alloys as baths for tempering tools. The article to be tempered is placed on the alloy, which is then heated until it melts. The article is then of the required temper, and is quickly cooled in water. Alloys of lead and bismuth have also been tried, but they are too easily oxidised, and have a great tendency to separate on cooling. Alloys of bismuth and tin succeed better. Those best known are