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THE Franklin Institute is not responsible for the state.
Mining and Metallurgical Section.
Stated Meeting, held Wednesday, November 13, 1901.
BY JOHN ALEXANDER MATHEWS, PH.D.
In our museums and collections may be found metallic
The art of refining in furnaces seems to have been known long before the Christian era. Pliny and Dioscorides give fairly accurate accounts of the smelting operations of their times, but do not explain or attempt to explain the chemistry of these processes. The behavior of mercury and gold—that is, the general facts concerning amalgamationwas known long before Pliny's time, and cupellation, as a refining process for gold, was certainly practiced before the second century B.C. The method of separating silver from gold by a process of cementation, in which the silver was “sweated out” by heating the gold silver alloy with salt and alum-shale, originated before the time of Christ, and is mentioned by Pliny.
Cupellation for the sake of quantitative estimation of gold and silver in alloys seems to have been first introduced by Roger, Bishop of Salisbury, in the reign of Henry I, early in the twelfth century. It must be remembered that in those days, and even for centuries later, the right to coin money was conferred by contract upon private individuals. It had also been the custom with those favored gentlemen to amass great fortunes by debasing the coinage of the realm and to escape punishment by occasional costly gifts to their king. When Roger applied cupellation as a method of assaying coins, the coiners were called to account, and of fifty examined, only four escaped punishments, which were doubtless more effectual than humane.
We have already mentioned bronze and gold-silver alloys; the cupellation process implies the existence of leadgold and lead-silver alloys. Tin, though of relatively early discovery, was known in its alloys still earlier than as a distinct metal. Its alloys with lead were used as solder by the Romans. Brass, the most useful of all alloys, save only steel, was described by Aristotle, but was not recognized as an alloy. The people of his time knew that when copper was fused with a certain earth (zinc ore) its color was changed. It became more yellow and golden in appearance, and this phenomenon doubtless had considerable influence in strengthening the faith of the alchemists in later centuries in the doctrine of the transmutability of metals.
In succeeding centuries, such metallurgists as Geber, Biringuccio and Agricola wrote, speculated and experimented much upon the phenomena of reduction and oxidation of metals. Nearly three hundred and fifty years ago the Royal Society took up the problem of the oxidation of molten lead in air, and discovered that its increase in weight was due to some constituent abstracted from the atmosphere. During all the centuries down to the phlogistic period, other metals were discovered and other alloys produced either by chance or design; and the profound changes which metals undergo when heated together were probably recognized by the shrewd observers of all ages. In the phlogistic period, which may be said to comprise the end of the seventeenth and most of the eighthteenth centuries, metallurgy had so advanced that a crude explanation of the distinction between wrought-iron, cast-iron and steel had been made. It does not appear, however, that any very decided opinions as to the constitution of metallic mixtures were advanced at this time and, indeed, one could scarcely expect that the profound questions of molecular physics, which a study of alloys involves, could have been solved or even approached before the announcement of a molecular theory of the constitution of matter, nor could it have made any progress earlier than the period of chemical reform instituted by Lavoisier, and properly known as the period of quantitative investigation.
Just at this time, however, when all conditions seemed favorable for investigating the molecular constitution of metals and alloys, Dalton announced the atomic theory and the law of simple numerical proportions. Thus, the great aim of the early chemists, that of determining the exact composition of substances, was directed toward bodies which were recognized as true chemical compounds whose composition might be expected to confirm Dalton's law. Alloys manifestly did not, in many instances, come under the provisions of the atomic theory, and hence were neglected by chemists and largely, too, by physicists. More recently the relations of physical properties to chemical constitution have attracted chemists and physicists alike, but in this