3. Mercury, zinc and tin. This amalgam is found the most effectual for the excitation of electric machines. Mr. Cuthbertson recommends 1 part zinc, 1 tin and 2 of mercury for the plate machine amalgam. But for a cylinder the best amalgam I have made contains more than twice the above portion of mercury. I form an alloy of 58 parts zinc and 52 tin, (2 atoms to 1). To this alloy I add 250 mercury, and fuse the mixture; the liquid mass crystallizes about 222° into a white, moderately hard amalgam. This is pulverized in a mortar and mixed up with ¹/₁₂ of its weight of hog’s lard. A small portion then is spread upon a piece of leather and applied to the machine when in action. It is probable however that a harder and less unctuous amalgam may be better adapted to the plate machine. This amalgam of mine consists of 4 atoms of zinc, 2 of tin and 3 of mercury.
I have tried the amalgams of zinc and tin separately and find that they answer for electric excitation as well as when combined. They ought to be formed of 2 atoms zinc and 1 of mercury (58 parts to 167), and of 2 atoms tin and 1 of mercury (104 parts to 167). If we choose to combine them, we have only to take 2 parts of the zinc amalgam and 1 of the tin amalgam and triturate them together.
Bismuth amalgam is not good for electric excitation; lead amalgam is better; but they are much inferior to those of tin and zinc.
Alloys of Copper with other Metals.
1. Copper and iron. These two metals may be united with difficulty by heat; but the compound possesses no useful property.
2. Copper and nickel. A white, hard, brittle alloy is said to be formed by combining these two metals. The alloy is scarcely known.
3. Copper and tin. The metals of copper and tin, may be fused together and united in almost any proportion by skilful treatment; but it is found that only a few of the proportions constitute alloys possessing properties eminently valuable to the arts.
The alloys of copper and tin are commonly called bell-metal; but they receive more particular names according to the purposes for which they are destined, as bronze, speculum metal, gun-metal, &c. those of them which are yellow are frequently confounded in common language with brass, as brass guns, &c. Indeed the ancient Greeks and Romans seem to have been in possession of these two alloys, under one and the same name. The χαλκος of the Greeks, being used for cutting-instruments, must have signified bell-metal, or the alloy of copper and tin as well as brass, as indeed is proved by the analysis of them. The æs of the Romans seems also to have included the same compound. Ancient copper coins too are usually found to contain tin.
Tin united to copper in certain proportions gives a surprising degree of hardness and tenacity to the alloy, much superior in these respects to either of the ingredients. In other proportions it makes the compound highly sonorous, as in bell-metal properly so called. Tin also increases the fusibility of the compound in proportion as it abounds, being itself fusible at the low temperature of 440° Fahrenheit.
The principal varieties in the alloys of copper and tin are enumerated below, beginning with those in which the copper is most abundant. The atom of copper is taken at 56 and that of tin at 52 weight, the hardness of these metals is denoted by 7.5 and 6 respectively, by Kirwan.