ducting power for heat and electricity, sound,
elasticity, and tenacity. Very few of the metallic elements are found pure in nature, but for the most part they are found alloyed with some other metallic element: thus gold usually contains traces of silver; copper is alloyed with silver or bismuth; lead almost always contains silver and frequently antimony; platinum occurs
as an alloy with iron, iridium, osmium, and
other metals. The great value of alloys in com-
merce is due to the fact that certain properties
which are desirable for practical purposes may
he imparted to many metals by a suitable addi-
tion of other metals. For instance, gold and
silver are too soft for use as coins when pure,
but may be rendered sufficiently hard by the ad-
mixture of small proportions of copper. Simi-
larly the hardness of copper is greatly increased
by the addition of zinc, yielding brass. In mak-
ing alloys, the least fusible metal is melted
fust, after which the others are added. When
three metals are used to form the alloy, they are
melted in pairs and afterward together. The
fused mass must be kept well stirred until the
mixture is complete, otherwise the tendency
would be for the heavier metal to sink to the
bottom and the alloy would not be of uniform
composition. The more important alloys, which
are included among the materials of constructive
engineering, are the bronzes, the brasses, the coin
alloys, and a few alloys of tin. lead, zinc, anti-
mony, and bismuth. All the other alloys are of
use for a few special purposes only. The following are the principal alloys, their composition and uses:
Bronze is an alloy of copper and tin. The knowledge of bronze is very old, it being used by the ancients for making coins, weapons, tools, and ornaments. Many of these ancient peoples were very skillful bronze founders. The principal bronzes are those used in coinage, in ordnance, in statuary, in bells and musical in- struments, and in mirrors and the specula of tel- escopes. Coin bronze as made by the Greeks and Romans consisted of from 96% copper and 4% tin, to 98% copper and 2% tin. Modern investigations have shown the range of good alloys for this purpose to be quite large, varying from 96% copper and 4% tin to 80% copper and 14% tin, the best falling near the middle of this range. Gun bronze has different compositions in different countries, but the most common pro- portion would seem to be 90% copper and 10% tin, or 89% copper and 11% tin. When well made it is solid, yellowish in color, denser than the mean of its constituents, and much harder, stronger, and move fusible than commercial cop- per; it is somewhat malleable when hot and much less so when cold. Statuary bronze is nearly the same composition as gun bronze. It should be rapidly melted, poured at a high temperature, and quickly cooled to get the best results. Bell metal is richer in tin than the preceding, and varies in composition somewhat with the size of the bell, the proportion of tin being the larger in the case of small bells. The range of good practice in bell metal is from 18% to 30% tin and from 82% to 70% copper. Chinese gongs are made from 78% to 80% copper and 22% to 20% tin, and are beaten into shape with the hammer, being tempered at intervals during the process. (See Annealing.) Bell metal is dense and homogeneous, fine ground, malleable if quickly cooled in the mold, rather more fusible
than gun bronze, but otherwise similar, excelling, however, in hardness, elasticity, and sonority. Speculum metal contains often as much as 33% tin; it is almost silvery white, extremely hard and brittle, and capable of taking a very perfect polish. Bronze for bearings and other friction surfaces in machinery is made of many proportions, varying from 88% to 96% copper, as more or less hardness is required. Phosphor bronze is a triple alloy of copper, zinc, and tin, which has been given exceptional purity by fluxing with phosphorus. It is very tough and hard, and is used for piston rings and valve covers, pinions, cog wheels, screw propellers, etc. Tobin bronze is an alloy of copper and zinc in the proportion of about 59 to 38, with small percentages of tin, iron, and lead. It has great tensile strength, and corrodes with great difficulty. Aluminum bronze consists of 90% copper and 10% aluminum, and is an exceedingly tenacious material. Manganese bronze is an alloy consisting of about 88 % copper, 1½% tin, 8.7% zinc, and smaller percentages of iron, lead, and phosphorus; it is much used for making screw propellers. Both Tobin bronze and manganese bronze are in reality more nearly brasses, since the zinc percentage is greater than the tin percentage.
Brass is an alloy of copper and zinc in about the proportions of copper 66%% and zine 33%%. Brass is extensively employed in the arts in the manufacture of scientific apparatus and mathematical instruments, the small parts of machinery, and many sorts of hardware. It is also drawn into wire, and rolled into sheets, and rods, which are used for a multitude of pur- poses. Brass is harder than copper, verj' malle- able and ductile, and can be "struck" in dies, formed in molds, or "'spun" in lathes into vessels, of a wide varietj' of forms. It is a much poorer conductor of electricity and heat than copper, and is more fusible. Aluminum brass is made of equal weight of aluminum bronze, copper, and zinc. It has a very high tensile strength, and has been used for screw propellers.
Other alloys than bronzes and brasses exist in an immense variety, and have numerous applications in the arts and sciences, although they are much less used than the bronzes and brasses. Only a few of these alloys can be mentioned here. German silver is an alloy of copper, zinc, and nickel in the respective proportions of about 60%, 20% and 20%. It is used for table utensils, ornaments, and in the form of sheets, and is one of the most difficult alloys to handle in the foundry and rolling mill. I'rirter is an alloy of tin and copper often mixed with lead. Britannia metal is an alloy of tin, antimony, copper, and brass. It and pewter are much used in making table utensils. Stereotype metal is an alloy of 16% antimony, 17% tin and 67% lead. (See Printing.) Babbitt metal is an alloy of 4 part copper, 12 parts tin, 8 parts regains of antimony melted together, and 12 parts tin added after fusion. It is used for lining bearings for journals. Holders are alloys used for joining metallic surfaces and parts, and have a wide range of composition. The soft solders arc made of tin and lead; the hard solders are usually made of brass, and special solders are composed of various alloys of copper, zinc, lead, tin, bismuth, gold, and silver. In making solders, great care has to be taken to secure uniformity of composition. For this reason, they are often granulated by pouring from a height into water, or by re-
