more promptly, giving rise to a frothy appearance, this is removed by rinsing the work in water, after which it is dipped in strong nitric acid for a few seconds, washed in water, and then washed in water containing dissolved argol, finally being dried in hot saw-dust. The argol solution is

said to prevent a brownish discoloration, or mottling of the surface, which would otherwise occur. In this last dipping it is important that each article should be dipped separately, and not a number strung together on wire, as is often the case in the former dippings.

CHAPTER III

BRONZE

§ 63. The term "bronze" will be applied in this work to all alloys consisting chiefly of copper and tin. These metals have been known from very remote times, and the importance of the mixture of copper and tin appears to have been among the first discoveries of the metallurgist. Instruments of various kinds were fabricated from these alloys, and weapons were made with a keen cutting edge, harder than iron, and almost rivalling that of steel. The bronzes of the ancients varied considerably in the proportions of the ingredients, for in the main copper and tin only were used, according to the purposes for which they were intended. Sometimes other ingredients were added, either purposely to produce a given effect, or it may be, in some cases, that bodies other than copper and tin were present as accidental impurities. This would arise from the use of impure metals, derived largely from ores of copper or tin associated with other ores, which is often the case.

Of late years very great attention has been devoted to the study of copper-tin alloys, and those proportions of the constituents which have been found by experience to give the greatest strength and the keenest cutting edge are the same as those used by the Greeks and Romans for their weapons of war and of the chase. The effect was produced by causing the bronze to undergo a process of hammering, as well as a method of hardening, by heating and slow cooling.

Many ancient coins were made of bronze, containing in some cases lead, zinc, or iron. The following table will show the composition of some ancient bronzes :

It will be observed from an examination of the foregoing table that the principal constituent of bronze is, in all cases, copper, the other components being added to harden or otherwise modify its properties, according to the purpose for which the alloy is intended. Tin has the property of

hardening copper, as already stated; the alloys are capable of taking a high polish; they present a beautiful metallic lustre, and with their moderate melting points, and fluidity when melted, form excellent alloys for casting. In certain proportions copper-tin alloys emit a beautifully clear sound when struck, the quality of which may be modified by slightly altering the composition of the mixture. Certain varieties of bronze containing, in addition to copper and tin, zinc, lead, manganese, iron, silicon, or phosphorus, are now largely manufactured for machine and engineering purposes.

The great feature of modern bronzes is the substitution of triple and quadruple alloys for the old dual alloys. French bronzes nearly always contain the four metals, copper, tin, lead, and zinc, and in some cases small quantities of nickel, arsenic, antimony, and sulphur. Each of these elements exerts an influence on bronze in proportion to the amount present, and if such influence is prejudicial for certain uses, care must be taken in the selection of the metals employed for admixture. Impure copper is by no means a rarity in commerce, and may contain ingredients fatal to the properties of certain varieties of bronze. The difficulty of preparing alloys of definite composition is increased when scrap is remelted with new metal, unless great care is taken to keep scrap of a given quality separate from other varieties; such old metal is also liable to contain iron and other foreign metals mechanically mixed with it.

Zinc in small quantity added to copper and tin has often a beneficial influence, as in casting, for instance, the metal runs thinner, fills up the moulds, and is freer from pin-holes. The zinc probably acts favourably in uniting with any oxygen which may be present, forming oxide of zinc. If the addition of zinc much exceeds what is required for this purpose, the alloy will be weaker, although harder, and the colour will more or less resemble that of brass. For this reason the amount of zinc should not exceed 2 per cent when high tenacity and elasticity are desired as important factors in the alloy.

Lead alloys very imperfectly with bronze, showing a great tendency to liquate out on cooling, the greater portion being found in the lower part of the casting. A small quantity of lead is said to make the alloy more malleable and denser. The peculiar patina of a velvety black colour found on old Chinese bronzes is probably due to the presence of lead.

Iron, in certain amounts, affects the properties of bronze very beneficially. It hardens the alloy and increases its resistance to wear in cases where the bronze is subjected to considerable friction, as in machinery bearings. Such alloys are paler in colour and more difficult to melt than with copper and tin alone. In small quantity iron increases the tenacity of bronze.

In 1858 Parker noticed that the addition of phosphorus during the melting together of copper and tin improved the physical properties of bronze in some respects, and this addition was eventually introduced into bronze manufacture with very successful results. The action of phosphorus in phosphor-bronze is to exert a refining influence on the mixed metals, rather than to form a definite alloy of copper, tin, and phosphorus, since many samples of phosphor-bronze of excellent quality contain but the merest traces of phosphorus. During the melting of copper and tin a certain amount of oxides is formed, which, being soluble in the molten metals, exerts a weakening influence on the alloy by preventing that intimate union of the constituent metals which is necessary to give the strength, toughness, and durability for which some varieties of bronze are noted.

Phosphorus has a strong affinity for oxygen, and when brought in contact with metallic oxides, such as those of tin and copper, reduces them, forming oxide of phosphorus. Now this oxide has an acid character, and readily unites with metallic oxides, which are generally basic, to form a fusible slag. This slag, being lighter than the metal and very fusible, floats on the surface and may be readily removed. If the requisite amount of phosphorus be added for the above purpose, the oxygen will be completely removed; if any