affected by such gases, provided it has the requisite strength and endurance. In short, the properties desired for good gun-metal are that it should be very tenacious, sufficiently hard and elastic to resist distortion, indifferent to the ordinary chemical influences, thinly liquid when melted, and capable of settling down solid in the moulds when cast.

Modern practice in producing gun-metal has resulted in the exclusive use of copper and tin, as combining the above advantages in the highest degree, and although the addition of a third metal may be advantageous in strengthening one particular property, it is injurious to the required properties taken as a whole. The addition of a little phosphorus is useful in special cases, but the quantity added must be very small. In fact the phosphorus is used only as a purifier, and the amount remaining in the bronze is almost infinitesimal, and cannot be reckoned as a factor in the final alloy. In old guns many foreign ingredients are found, such as nickel, cobalt, lead, iron, bismuth, arsenic, etc.; but in many cases the amounts are so small that they may be looked upon as adventitious impurities, and not as purposely introduced into the mixture for alloying. With regard to lead, zinc, and iron, these metals have doubtless been intentionally added in certain cases, with a view to producing a given effect. The other metals mentioned are occasional impurities found in commercial copper, which in former times was much more impure than the varieties obtainable at the present time.

The term "gun-metal," as understood at the present time by many engineers and brassfounders, is not confined exclusively to alloys of copper and tin, as zinc and other metals are very frequently added; and, in fact, the term seems to be applied to any alloy in which copper largely predominates, and which possesses considerable strength and toughness. Several of the triple alloys of copper, tin, and zinc possess many excellent properties, which fit them for parts of machinery and for anti-friction metals. These will be referred to under the head of "Machine Brasses." The following table will show the composition of a few of these alloys :

No. II is

Nos. III and No. V is hard but

No. I is tough, malleable, and tenacious. hard, somewhat unyielding, and easily broken. IV work well under the file and chisel. somewhat malleable. No. VI is hard and resisting, tough, and works fairly well with the file and chisel. No. VII is hard and easily broken, but may be filed.

The alloys are hard and brittle when the copper is less than 66 per cent of the mixture; and when the copper is reduced to 50 per cent the alloys are extremely hard and brittle. The addition of a little lead improves the above alloys for turning and filing. A sample of so-called "gunmetal," stated by the user to be very strong and durable, and used for crown-wheel escapements, gave on analysis—

An alloy prepared by Mr. Stirling, and tried in the Arsenal of Woolwich, has a resistance to flexion much greater than that of ordinary bronze; it contains

The above alloy is difficult to obtain in a sound and homogeneous state.

BELL-METAL

§ 69. The various alloys used in the manufacture of bells consist essentially of copper and tin, but in some cases other metals are added in small quantity either for cheapness, or to produce a desired quality of sound. The additional metals chiefly used are zinc, lead, iron, and sometimes bismuth, silver, antimony, and manganese. The following table will show a few of the proportions employed:

In addition to the above alloys, small bells are made of ordinary brass, especially when a wrought pattern is required on the outside for the purpose of ornamentation; but the sound from such bells is of inferior quality. Mr. P. M. Parsons states that manganese-bronze is peculiarly adapted for large bells. The advantages claimed are, that bells cast from it possess the same or greater sonorousness, with a more mellow tone, and are at the same time so tough that they cannot be cracked like bells made of ordinary bell-metal, which is made brittle in order to acquire the requisite sonorousness. Good bell-metal should give a pure full sound, the quality of which varies with the purpose for which it is designed. The sound may be modified by chemical composition, but it is also dependent upon the mode of manufacture. The alloy should be hard, homogeneous, fine-grained, and strong. The colour of bell-metal of normal composition, containing from 76 to 80 per cent copper and 24 to 20 per cent tin, is yellowish-gray, and when very slowly cooled from a red heat is very hard, difficult to file, sonorous, brittle, and exhibits a fine-grained fracture when broken. When heated to redness and suddenly cooled by plunging into cold water it becomes moderately soft, and capable of being easily filed, turned, and otherwise worked. It may be hardened by heating to redness and allowing to cool slowly. At a temperature not far below redness it is malleable. An alloy of 80 per cent

copper and 20 per cent tin, when slowly cooled after fusion, exhibits a dingy-gray striated appearance and is very brittle. If suddenly chilled in cold water from a low red heat it becomes yellowish-gray and extensible. During the ignition, if the temperature be raised too high, white globules of an alloy, rich in tin, separate out, so that bell-metal appears to be resolved into two alloys at a temperature below its melting point, which on slow cooling reunite and form a brittle alloy, but remain separated if the cooling be too sudden.1

1 Watt's Dic. vol. ii. p. 44.

The tone of a bell is influenced materially by its size and shape, by the thickness of the metal, and by the ratio of the height to the diameter. The skill of the bellfounder is not only exercised in finding the best composition to be employed, but also in determining the exact shape and size for a required note and tone, which is of special importance in chimes. Some dinner-bells are made of two halves, having a clapper on each side, the two having different sounds, of a major or minor third interval, and thus forming an agreeable combination when struck.

Table bells are sometimes preferred to be white, which is effected by casting in white metal, as given in the preceding table, or the ordinary alloys are whitened by the addition of tin or nickel. For boiling white, the bells are first highly polished and then placed in a hot bath of water with cream of tartar added, and a layer of granulated tin placed between each row of bells in the bath. The bath is kept at a temperature of 212° F. for two or three hours, then the bells are removed, washed in clean water, and finally polished with chamois leather. Many bells are now nickel plated.

The melting and casting of bell-metal is similar to that of bronze. The copper is generally melted first, the tin subsequently added, and the whole vigorously stirred to promote intimate mixture. When scrap is used along with new metal, the copper and scrap are melted together, and the new tin added as before. In casting small bells no oddside is required as in ordinary brass casting. The patterns, with the convex side upwards, are placed on a board, and a casting frame placed over them; parting material (generally brick-dust) is then dusted over them to prevent the casting sand sticking to the patterns. A layer of raw sand, pounded very fine in a mortar, is then laid on the patterns as a first coat, and the frame filled up with ordinary sand. The sand is well rammed down, first with the hands, then with a mallet, and sometimes with the feet. A board is then placed on the top, and the frame inverted; one side of the mould is now completed. The peg frame of the mould is