For white alloys for bearings see p. 229.

Different manufacturers hold different views as to the best mixtures to employ for a given purpose, as will be seen by the very varied proportions given in the previous table for bearings. Moreover, an alloy used for a special purpose will not possess the same properties under all circumstances, and the mixture must be modified to suit local requirements. It has been shown in the preceding pages how greatly even small quantities of impurities will affect metals, so that an alloy containing foreign ingredients will have its molecular structure disturbed by their presence, and this has doubtless led some persons to alter their mixtures, when they should have looked to the purity of the constituent metals employed. Even chemical analysis, however, will not always reveal the reason why a certain alloy possesses certain properties, as the qualities may have been conferred by special mechanical treatment. The mode of melting, mixing, and casting will also influence the final result.

The following table is intended to show the composition of various alloys, and may serve as a general guide :

:

The following tables represent the mixtures employed by a large engineering firm, using scrap and new metal :

§ 73. This is an alloy of copper and tin containing a small quantity of phosphorus and other ingredients in definite proportions, the general properties of which have been already discussed, so that the mode of manufacture and the suitability of the metal for machinery and engineering purposes need only be here stated. It is prepared by melting and mixing copper and tin in the usual way, and adding a certain quantity of phosphor - copper, or phosphor-tin, or both, and well incorporating the, ingredients

by vigorous stirring. A new plumbago pot is used so as to avoid contamination from other metals, and charcoal or coke added as a covering to prevent unnecessary oxidation. For large castings the moulds are thoroughly dried and dressed with a mixture of blacklead and water. Small work is cast in ordinary green sand. In cooling from the molten state it passes directly from the liquid to the solid condition, without passing through an intermediate pasty state. If remelted it does not sensibly alter in composition, except that when phosphorus is present in notable quantity, that element is slightly decreased by volatilisation. If the alloy be poured into the moulds at too high a temperature, a certain amount of separation of the constituents occurs as in ordinary bronze, so that it is advisable to pour phosphor-bronze only just before the setting takes place. This is effected by adding ingots or runners to the molten metal, and when the metal no longer melts these, but adheres to them, it is a sign that pouring should take place.

For rolling; drawing into wire, rods, and tubes; making bolts, springs, screws, etc., the tin should not exceed 4 to 5 per cent, and the phosphorus less than per cent, the remainder being copper. It can be forged into firearms of various descriptions.

For pinions, valves, steam and boiler fittings, pumps and general ornamental castings the tin is about 7 per cent, and the phosphorus varies between 15 and 25 per cent. This is a strong, tough metal, and much harder than the preceding alloy.

For axle-bearings, slide-valves, bushes, cog-wheels, and all parts of machinery exposed to much friction, a metal of great hardness and strength is required, and should contain from 90 to 91 per cent of copper, 9 to 8 per cent of tin, and from to 1 per cent of phosphorus.

When the amount of phosphorus is much greater than the above, the alloys are harder, less malleable and tough, and in cases where hardness is the chief requisite, can be made to rival steel in this respect. With upwards of 4 per cent phosphorus the bronze is useless.

Phosphor-bronze possesses the advantage of not becoming crystalline under the action of repeated shocks and bends, and is therefore well adapted for wire rope. It resists the action of sea-water better than copper, and also the corrosive action of water in mines better than iron or steel.

The following table published by the Phosphor-Bronze Company shows the results obtained with various axlebearings, but as the compositions of their phosphor-bronzes are omitted, the table is robbed of its chief value as a standard of comparison.