that Australian or English B.S. copper be used. Unannealed, in rods or sheets of moderate thickness, the metal has a tensile strength of 67,000 to 72,000 lbs. per square inch, according to the amount of rolling it has received.

The composition of brass for ordinary wire drawing varies from 67 to 72 per cent copper, and 33 to 28 per cent of zinc. It has been stated that a little lead is sometimes added to brass intended for rolling, but this is not admissible in brass intended for wire drawing, as it diminishes the tenacity; and the ductility of a metal largely depends upon its tensile strength. Wire-drawers state that if brass wire is annealed immediately it has been taken off the drum, on which it has been wound during the process of wire drawing, it will fly to pieces. This effect is prevented by subjecting the coil of wire, after its removal from the drum, to strong concussion, by seizing one part of the coil with the hands, raising it, and heavily striking a bench with the other part, repeating the process several times.

Some kinds of brass wire undergo a molecular change in process of time, especially if subjected to vibration, and become extremely brittle. Brass chains, used to support objects, such as chandeliers, etc., have been known to lose their tenacity, become brittle, and break without any apparent cause. Ship bolts of Muntz' metal are sometimes found to undergo a singular kind of exfoliation, the metal on the exterior becoming separated, more or less completely, into concentric laminæ, from a solid cylindrical nucleus within.1

CAST-BRASS

$29. The composition of brass used for castings varies considerably, and depends to a large extent upon the uses to which the cast articles are to be put, as also with the particular shade of colour it is desired to obtain. Thus various alloys for jewellery, having a reddish-yellow colour, are composed of from 82 to 90 per cent of copper, while those

1 Percy's Metallurgy, p. 621.

containing 60 to 70 per cent copper are of a full-yellow colour. The composition most generally used consists of 66.6 per cent copper and 334 per cent zinc, which is termed English standard brass. It works excellently under the rolls and hammer, and may be used also for wire drawing. Cast-brass generally contains more zinc than that which is to be worked into sheet or drawn into wire and tubing; it is therefore more fusible. Moreover, the materials are not selected with such care as is the case with those for rolling and wire drawing, so that cast-brass is often more impure than rolled brass, a large quantity of scrap being generally melted with new metal. The most common impurities are lead, tin, and iron. These metals are not always a disadvantage, for lead is generally added to the extent of from 1 to 2 per cent in brass required for turning and filing, as it gives sharpness to the metal and prevents fouling of the tools. The presence of a little tin is an advantage when hardness is requisite, as in the case of bushes, for example. A little iron hardens brass, and tends to increase its tenacity and elasticity. But it is not advisable to introduce metals other than copper and zinc, unless for specific purposes, as mentioned above. It should be borne in mind that however beneficial the addition of lead, tin, or iron may be in special cases, the simultaneous presence of two or all of these metals is generally injurious, whatever the action of one of them may be when present alone in the brass. In fact, it may be stated as a general rule, not without exceptions, that an alloy of two metals is more stable than that of three or more; and the greater the number of metals alloyed together, the greater will be the difficulty in obtaining uniform and sound castings. Lead especially has a tendency to separate out from the mixture in cooling, producing unsightly black spots, especially when present to the extent of 2 or more per cent.

When the metal is cast in a large mass, or the castings are of considerable size, it is found that as the metal solidifies, a greater proportion of copper is found in the under portion than in the upper part of the casting, and the alloy is thus

liable to be deeper in colour below than above. The constituents of an alloy of two or more metals tend to separate according to their respective densities, and the more numerous the constituents, and the greater the difference of their specific gravities, the more marked will this tendency be; but when the metals are in chemical combination with each other, no such separation will take place.

To test the difference in composition at top and bottom of cast-brass the two following experiments with the accompanying analyses were made by the author. Different varieties of brass were remelted together, so as to form two different qualities of brass, and cast into upright tapering iron moulds 4 inches high and about 2 inch square.

TABLE OF DIFFERENT QUALITIES OF ORDINARY CAST-BRASS AS DETERMINED BY ANALYSIS OF COMMERCIAL SAMPLES

Brass employed for the production of fine castings requires to possess other properties than those of being able to be filed and turned in the lathe. It must be thinly liquid when melted, and not in any degree pasty, so that it will readily flow into the minutest and most intricate crevices of the mould, and the texture must be fine-grained and uniform throughout. Moreover, as such castings specially require to be sound and free from pinholes, it must be capable of remaining thinly liquid until near the point at which it solidifies, so that the metal may be poured at the lowest possible temperature, thus enabling air and other gases, absorbed during the melting, to escape as much as possible before pouring. When the castings have to be subsequently gilded, it has been found that when the metal is similar in colour to gold, it requires less gold to produce the desired effect than when the brass is of a pale yellow colour. For this reason brass of reddish-yellow colour is preferred. The French have brought the casting of fine articles, as well as larger articles, to a high degree of perfection; clock-cases, statuettes, and other artistic ornamental work are manufactured by them on an extensive scale. The alloys used for these purposes are often called bronze, but they are generally brass, with the addition of a little lead and tin, which impart to the metal a bronze-like colour. The following are the proportions used for a few of the French mixtures :

The above alloys can be readily cast, worked with files and

turning tools, and readily gilt.

These alloys are distinguished by a beautiful gold colour which very closely resembles that of gold alloys. They are very ductile and tenacious, can readily be stamped and rolled, and admit of a fine polish. According to an old recipe orëide is prepared in the following manner: "Melt 100 parts of copper and add, with constant stirring, 6 parts of magnesia, 3.6 parts of sal-ammoniac, 18 parts of lime, and 9 parts of crude tartar. Stir again thoroughly, and then add 17 parts of granulated zinc, and after mixing it with the copper by vigorous stirring, keep the alloy liquid for one hour. Then remove the cover of dross, and pour out

the alloy."

§ 31. Talmi or Talmi Gold.-Also termed Abyssinian

Talmi gold is the name given by the French, who use the foregoing alloys for cheap jewellery, which is distinguished by good workmanship, great durability, and a beautiful golden colour. The colour is retained for a considerable time, as it possesses a thin coating of gold, firmly welded to the alloy by rolling, and when the gold is of sufficient thickness the characteristic appearance may be retained for years without alteration. But many cheap imitations

are manufactured and sold under this name which consist of common brass, gilded with a thin coating of gold.