The following are some of the most important alloys which have yet received an extensive appli cation in the arts and manufactures.
Aluminium Bronze.—This alloy is composed of 90 parts of copper and 10 parts of aluminium. It is a definite chemical compound, and was discovered by Dr. Percy. It was manufactured for many years at Washington, near Newcastle, by J. L. Bell, who obtained it by melting the copper in a crucible made of plumbago or some other highly refractory material; the correct proportion of metallic aluminium was added to the melted copper, the two .metals uniting with evolution of intense heat. Aluminium bronze is of a yellow colour, resembling gold ; it is extremely hard and tenacious, and possesses great malleability and strength. It is admirably adapted for the workiog parts of machinery where great durability is required, and has a power of withstanding compression nearly equal to that of the best steel. Aluminium bronze containing 10 per cent. of aluminium possesses the maximum degree of hardness, strength, and tenacity ; a larger proportion of aluminium renders the alloy weak and brittle. It has a specific gravity of 7.68 ; the weight of a cubic inch is lb., and the tensile strength 32 tons per square inch.
Bell-metal.— An alloy of copper and tin in proportions varying from 3 to 5 parts of copper to 1 part of tin. It is of a yellowish-grey colour, hard, brittle, and sonorous, and exhibits a fine-grained fracture. Cooled suddenly from a red heat, it becomes soft, but regains its hardness after being re-heated and cooled very slowly. Small house-bells are usually made of an alloy of 2 parts of copper with 1 part of tin ; but for larger bells a higher proportion of copper is needed.
The larger the proportion of copper in the alloy, the deeper and graver is the tone of the bells formed from it. The addition of tin, iron, or zinc causes them to give out a sharper tone. Where the quality of the tone is the chief object sought after, care must be taken to employ only commer cially pure copper. The presence of lead, even in very small quantities, affects prejudicially the sonorousness of the alloy. Silver, on the contrary, is said to give sweetness to the tone. The pre sence of this metal has been detected in many old church bells, which, according to tradition, were cast from crucibles into which articles of silyer had been thrown as votive offerings.
The composition of some varieties of bell-metal is shown below : (1) Copper, 39 parts ; tin, 11 parts. This is the most sonorous of all the alloys of copper and zinc. (' standard.') (2) Copper, 77 parts ; tin, 21 parts ; antimony, 2 parts. Paler, and inferior to the above. (' Founders' Standard.') (3) Copper, 4 parts ; tin, 1 part. Very deep-toned and sonorous.
(4) Copper, 3 parts ; tin, 1 part. Used for church and other large bells.
(5) Copper, 17 parts ; tin, 8 parts. Best proportions for house-balls, hand-bells, &c. (G) Copper, 72 parts; tin, 26j parts; iron, 1} parts. Used by the Paris houses for the bells of small clocks.
is perhaps the most useful and important alloy known. Its composition varies widely with the uses for which it is intended, but its constituents are copper and zinc, usually in the proportions of nearly two parts of the former to one part of the latter. Brass may also eoutain small quantities of tin and lead. The qualities which render this alloy so valuable may be briefly enumerated as follows :—It is harder than copper, and consequently better able to resist wear and tear. It is very malleable end ductile, and therefore admits of being either rolled into thin sheets,
shaped with the hammer, drawn into fine wire, or raised by stamping into objects of various forms. It is readily fusible, and therefore easily cast at a lower temperature than copper. It resists the influences of the atmosphere better than copper, although, if unprotected by lacquer or varnish, it rapidly tarnishes and blackens on exposure to the air. Filially, brass has a fine yellow colour, and is capable of receiving a beautiful polish.
The malleability of brass varies with its composition and with its temperature; it is also affected, to a sensible degree, by the presence, even in minute quantities, of certain other metals. Some varieties of brass are malleable only when cold, others only when hot, and others, again, aro never malleable. At a temperature just below its fusing point, brass, like copper, is brittle, and may be powdered in a mortar. Alloys of copper and zinc present a great variety of colour, ranging between the reddish hue of the former and the bluish-white of the latter ; the transition is gradual, and passes through all the intermediate stages of yellow. The following table represents the intensity of colour, hardness, and fusibility pesocQsed by these different alloys:— During the process of stamping brass, it must be hardened or tempered from time to time. At the end of the process it has lost its colour, owing to the formation of a coating of oxide during the tem pering operations. This coating is easily removed by plunging the metal into nitric acid, and then washing it thoroughly with water. A brilliant metallic surface is thus produced, ready to receive the customary layer of lacquer or varnish. This cleansing process is known as " dipping." If the brass contain any impurities, dipping will not impart to it a brilliant surface. The colour produced by dinning varies according to the strength of the acid ; this is due, it is believed, to the fact that the metals constituting the alloy are acted upon to a greater or less degree by acids of different degrees of dilution. The operation of dipping is performed in the following way :—The object, coated with a black coat of oxide, is plunged into nitric acid containing 1 part of the pure acid to 7 or 8 parts of water. It is allowed to "pickle," as it is termed, in the acid solution until the crust can be detached by rubbing the surface of the metal gently with the finger, when it is withdrawn, and washed immediately in water. It is next dipped into a much stronger acid solution, where it remains until the "curd" appears, or until the surface of the metal is entirely covered with minute bubbles of gas. This solution should be about twice as strong as the one previously used. The brass must then be washed with a plentiful supply of water, and roughly dried in cold sawdust. It is afterwards clipped, with the particles of wood still adhering to its surface, into strong nitric acid, where it remains only a few moments, then rinsed with a little water, and immediately afterwards thoroughly washed with water containing argol in solution. It is finally dried in hot sawdust, after which the surface is ready for the lacquer or varnish.