LIST OF AUTHORITIES REFERRED TO IN PRECEDING TABLE.

Bo.-Bolley.

1869, pp. 345, 348.

Essais et Recherches Chimiques, Paris,

Cr. Croockewit. Erdmann's Journal, 1848, vol. xlv. pp. 87-93.

C. J.-Calvert and Johnson. "Specific Gravities," Phil. Mag. 1859, vol. xvii. pp. 114-121; "Heat Conductivity," Phil. Trans. 1858, pp. 349-368.

De.-S. B. Dean. "Ordnance Notes," No. XL. Washington, 1875.

La.-Lafond. Dingler's Journal, 1855, vol. cxxxv. p. 269.

Ml.—Mallet. Phil. Mag. 1842, vol. xxi. pp. 66-68. Ma.-Matthiessen. Phil. Trans. 1860, p. 161; ibid. 1864, pp. 167-200.

Mar.-Marchand and Scheerer. Journal fuer Praktische Chemie, vol. xxvii. p. 193 (Clark's Constants of Nature). Mus. "Alloy."

Musschenbroek.

Ure's Dictionary, Article

Ri. Riche. Annales de Chimie, 1873, vol. xx. pp. 351-419.

66

U.S.B.- Report of Committee on Metallic Alloys appointed by United States Board, to test Iron, Steel, etc." T.-Thomas Tomson. Annales de Chimie, 1814, vol. lxxxix. pp. 46-58.

W.-Watt's Dictionary of Chemistry.

"Report on

Wa.-Major Wade, United States Army. Experiments on Metals for Cannon," Phil. 1856. We.-Weidemann. Phil. Mag. 1860, vol. xix. pp. 243,

244.

Notes on Table.—In the foregoing table the figures of order of ductility, hardness, and fusibility are taken from Mallet's experiments on a series of sixteen alloys, the figure 1 representing the maximum and 16 the minimum of

the property. The ductility of the brittle metals is represented as 0.

The relative ductility given in the table of the alloys experimented on by the U.S. Board is the proportionate extension of the exterior fibres of the pieces tested by torsion, as determined by the autograph strain diagrams. It will be seen that the order of ductility differs widely from that given by Mallet.

The figures of relative hardness, on the authority of Calvert and Johnson, are those obtained by them by means of an indenting tool. The figures are on a scale in which castiron is rated at 1000. The word "broke" in this column indicates that the alloy opposite which it occurs broke under the indenting tool, showing that the relative hardness could not be measured but was considerably greater than that of cast-iron.

The figures of specific gravity show a fair agreement among the several authorities in the alloys containing more than 35 per cent of tin, except those given by Mallet, which are in general very much lower than those by all the other authorities. In the alloys containing less than 35 per cent of tin there is a wide variation among all the different authorities, Mallet's figures, however, being generally lower than the others. Several of the figures of specific gravity have been selected from Riche's results of experiments on the effects of annealing, tempering, and compression, which show that the latter especially tends to increase the specific gravity of all the alloys containing less than 20 per cent tin to about 8.9. This result is due merely to the closing up of the blowholes, thus diminishing the porosity. The specific gravity of 8.953 was obtained by Major Wade by casting a small bar in a cold iron mould from the same metal which gave a specific gravity of only 8.313 when cast in the form of a small bar in a clay mould. The former result is exceptionally high, and indicates the probability that every circumstance of the melting, pouring, casting, and cooling was favourable to the exclusion of the gas which forms

blowholes, and to the formation of a perfectly compact metal.

The figures of tenacity given by Mallet, Musschenbroek, and Wade agree with those found in the experiments as closely as could be expected from the very variable strengths of alloys of the same composition which have been found by all experimenters.

Mallet's figure for copper 24.6 tons, or 55,104 lbs., is certainly very much too high for cast copper; the piece which he tested was probably rolled or perhaps drawn into wire. Haswell's Pocket-book gives the following as the tensile strength of copper; the names of the authorities are not given :

The strength of gun-bronze, as found in the guns, is not given in the table, which is designed to compare the various authorities on the tenacities of the alloys only as cast under ordinary conditions, and not when compressed, rolled, or cast under pressure.

N

COMMERCIAL VARIETIES OF BRONZE

§ 68. Gun-metal.—With regard to this alloy it should be stated that iron and steel have almost entirely superseded it for making guns, yet a consideration of its properties for this purpose may not be out of place in this work. For the construction of ordnance it is necessary to employ a metal which is capable of resisting great and sudden pressures, for it is calculated that in firing an ordinary cannon a pressure of 2000 atmospheres is suddenly developed; and it will readily be seen from this that a metal of peculiar strength and endurance is required for such a purpose. A vast number of experiments have been made to endeavour to arrive at the best proportions of copper and tin to employ in making an alloy for guns, but no uniform standard has been adopted for all countries, each preferring a separate mixture. It should be borne in mind that two alloys of exactly the same chemical composition may have very different properties conferred upon them by different mechanical treatment, and unless we know the manner in which a gun is finally completed, we cannot produce an article of similar properties by simply using the proportions given by chemical analysis. This is probably the reason why, in the guns of different countries and of different periods of history, a uniform standard has not been employed. The following table will illustrate this point :—

The proportions best suited for guns vary from 8 to 11 parts copper to 1 of tin; the best of these proportions, according to Karsten, is 9 parts copper to 1 of tin. alloy of 11 parts copper to 1 of tin appears uniform after sudden cooling to the unassisted sight, but when examined with a lens it appears to be composed of striated faces of a reddish alloy, mixed with a white one. If it be still more rapidly solidified by pouring into thick iron moulds, an alloy is obtained which appears perfectly uniform, even under the lens. When cooled in water, after continued strong ignition, it remains uniform; but if suffered to cool slowly after continued ignition, it becomes variable in composition, like that which has been slowly cooled after fusion. Hence the alloy which is uniform at the melting point, and likewise at a strong red heat, separates into two different alloys when slowly cooled. The large mass of a cannon cannot be cooled, even by moulds which conduct heat well, suddenly enough to prevent the separation of two distinct alloys, the one that is richer in copper solidifying first, while that which is richer in tin containing 82.3 per cent copper and 17.7 per cent tin, partly rises to the top and partly sinks into the mould.

Gun-metal must possess a considerable degree of hardness and elasticity, as in firing the gun the cartridge strikes several times against the sides, and if the metal yields permanently to the pressure thus exerted upon it, the bore gradually loses its cylindrical form after a time, and its accuracy for shooting at a given object is largely destroyed. Moreover, in firing a cannon a large quantity of gas is generated, which has a more or less corrosive action on metal, so that it is advisable to use that alloy which is least