ledge of practical metallurgy, his views as to this particular phenomenon were hardly in advance of Geber's; but we may claim Biringuccio as an early metallurgist, who knew the facts. and recognised that they were theoretically important. Cæsalpinus, in his work, De Metallicis, showed that the film which covers lead exposed to moist air and augments its weight, is due to an aëriform body. It was not till nearly a century later (1630) that a French chemist, Jean Rey,* stated that the increase in weight came from the air. The problem attracted much attention in England, and it is not a little interesting that among the very first experiments recorded by our own Royal Society, is a metallurgical series relating to the weight of lead increased in the fire on the " copels" at the assay office in the Tower, the account being brought in by Lord Brouncker in February, 1661.† Subsequently, in 1669, John Mayo showed that the increase in weight of calcined metals was due to a "spiritus," or distinct constituent of ordinary air. Nevertheless, Boyle heated lead in a small retort, and attributed the increase in weight, as Lemery also did,|| to his having "arrested and weighed igneous corpuscles." One of the most curious passages known is in the Hippocrates Chemicus of Otto Tachen, a German who lived at Venice, and published his book there in 1666. He describes how lead, when burnt to minium, increases in weight. This increase he ascribes to a substance of acid character in the wood used for burning, and then, by a very ingenious course of argument, based on the saponifying powers of litharge, makes out that lead is of the nature of, or contains an alkali which combines with, the "occult acid of the fat." This is a curious anticipation of a very modern classification, which brings lead into relationship with the alkalies and alkaline earths, as well as of Chevreul's investigations on saponification. Casalpinus had previously called lead "a soap" which in cupellation washes gold and silver. It is hardly necessary to point out how important this calcination of lead was considered by those who defended the Phlogistic theory in regard to chemical change, the theory propounded by the metallurgist Becker, which, for more than a century, exerted so profound an influence on scientific thought. His views were first embodied in the Physica Subterranea (1669) and in the Alphabethum Minerale (1682). Essais de Jean Rey (reprinted in Paris, 1777), p. 64. Tractatus quinque Medico-Physici, p. 25 et seq. (Oxonii, 1674). § Collected works, vol. ii. (1744), p. 395, and vol. iii. p. 347. According to his still more famous pupil Stahl, the litharge produced by the prolonged calcination of lead in air, is lead deprived of its phlogiston; but he and his followers were indifferent to the fact that when lead is burnt the weight of the resulting mass is greater than that of the original metal, and were content to insist that the burnt lead had lost its inflammable principlethat is, Phlogiston. Tillet, assayer of the Paris Mint, made some quantitative experiments which led up in a singular way to the work of Lavoisier, who, as is well known, overthrew the old phlogistic theory by showing that a chemical combination takes place, resulting in an augmentation of weight which represents the exact weight of the gaseous body added. At the same time it should be remembered that the phlogistic chemists made a great step in advance, as was admitted by J. R. Mayer* in his memoir on the mechanical theory of heat; and Odling, discussing the experiments on the oxidation of lead, has pointed out † that an error has arisen in consequence of the same word being used in a different sense at different periods of time; chemists, in fact, now substitute the words potential energy for phlogiston, or, as Dr. Crum Brown well observed, we recognise "that no compound contains the substances from which it was produced, but that it contains them minus something. We know now what this something is, and can give it the more appropriate name of potential energy; but there can be no doubt that this is what the chemists of the seventeenth century meant when they spoke of phlogiston." It will thus be evident that the main aim of chemical investigation down to the end of the last century was the explanation of calcination, combustion, or oxidation, and that lead was especially useful in solving the problem. It might, perhaps, be added that the absorption of oxygen by molten litharge furnished Ste. ClaireDeville,s a physicist and metallurgist, with an important step in the argument as to dissociation, thus connecting the history of the metal, lead, with the great advance on the borderland of chemistry and physics which has been made in modern times. The above remarks will be sufficient to show that conclusions of the utmost importance in the history of chemical theory were based on the very ancient metallurgical process of cupellation of lead, a process which affords an appropriate illustration, because, Bemerkungen über die Kräfte der unbelebten Natur, Liebig's Ann., vol. xlii. (1842). p. 233. + Proc. Roy. Inst., vol. vi. (1871), p. 323. Edin. Roy. Soc. Proc., vol. v. (1866), p. 328. Leçons sur la dissociation, and Wurtz, Dictionnaire de Chimie, vol. i. (1868), p. 1174. in the gradual development of the knowledge derived in the first instance from the metallurgy of lead, there is much that is typical of the mutual relation of theory and practice that still prevails. Now, as in the past, in the study of metallurgy, a prominent position must be given to the production of high temperatures, as it will be obvious that metallurgists have principally to consider the reactions of the elements when under the influence of heat. In the first half of the present century, temperatures higher than the melting point of zinc had not been determined with any degree of certainty; but, in 1856, Henri Ste. Claire-Deville pointed out that chemistry at high temperature, that is to say, up to the blue-white heat at which platinum volatilises and silica fuses, remained to be studied, as under such conditions ordinary chemical reactions may be modified or even reversed. Subsequently, in conjunction with Troost, he gave certain fixed points, such, for instance, as the boiling-points of cadmium and zinc; and his researches on dissociation have entirely modified the views generally entertained in regard to the theory of combustion. Indeed, so much is due to this illustrious teacher, that the best homage that can be offered to his memory will be to work in the directions he has indicated. Deville's experiments on dissociation have rendered it possible to extend to the groups of atoms in chemical systems the laws which govern the fusion and vaporisation of masses of matter, and this has produced a revolution comparable in its importance to that which followed the discovery of the law of definite proportions, for dissociation has shown us that true causes of chemical change are variations of pressure and of temperature. For instance, oxygen may be prepared on an industrial scale from air by the intervention of oxide of barium heated to a constant temperature of 700°, provided air be admitted to the heated oxide of barium, under a pressure of 1 atmospheres, while the oxygen, thus absorbed, is evolved if the containing vessel be rendered partially vacuous. It will be evident, therefore, that at a certain critical temperature and pressure the slightest variation of either will destroy the equilibrium of the system and induce chemical change. It will be clear that the measurement of high temperatures has become a question of much moment, and in this direction remarkable progress has recently been made. The essential difference in the properties of metals produced by a small quantity of foreign matter introduces one very distinctive feature of metallurgy-the enormous influence exerted on a large mass of metal by a "trace" of another metal or metalloid, that is, by a quantity so small that it appears to be out of all proportion to the mass in which it is distributed; and it may safely be asserted, that in no other branch of applied science has the operator to deal with quantities that are at once so vast and so minute. It may be that the "trace" is alone of value, as, for instance, the few grains of gold that can be profitably extracted from a ton of material, which, though containing only one part of gold in five millions by volume, is thereby entitled to be regarded as an auriferous deposit that can be profitably worked; or it may be that the presence of a minute percentage of a metalloid is prejudicial and must be extracted, in order that the physical properties of the remaining mass of metal may not be such as to render it useless. Due prominence is given to such facts in the following pages. It is assumed throughout that the student possesses a certain amount of chemical knowledge, but it will be evident that Metallurgical Chemistry is a special branch of chemical science which does not come within the ordinary sphere of the academic teaching of chemistry. It is often urged that metallurgical practice depends upon the application of chemical principles which are well taught in every large centre of instruction in this country, but a long series of chemical reactions exist which are of vital importance to the metallurgist, though they are not set forth in any British manual of chemistry, nor are dealt with in courses of purely chemical lectures. It is well to insist upon this point, because purely analytical and laboratory methods are so often given in the belief that they are applicable to processes conducted on a large scale and at high temperatures. It is urged that technical instruction should be kept apart from scientific education, which consists in preparing students to apply the results of past experience in dealing with entirely new sets of conditions, but it can be shown that there is a whole side of metallurgical teaching which is truly educational, and leads students to acquire the habit of scientific thought as surely as the investigation of any other branch of knowledge. It is, in fact, hardly possible in a course of theoretical chemistry, to devote much attention to specific cases of industrial practice in which reactions are incomplete because they are limited by the presence of bodies that cannot be directly eliminated from the chemical system. Take, for instance, the long series of reactions, studied by Plattner, who published the results of his investigations in his celebrated treatise, "Die Metallurgische Röstprozesse," Freiberg, 1856. A complex sulphide, of which copper is the main metallic constituent, contains some fifty ounces of silver to the ton, and the problem may be supposed, for the present, to be limited to the extraction of the precious metal from the mass in which it is hidden. The student deriving his knowledge from an excellent modern chemical treatise would find the case thus stated: "Ziervogel's process depends upon the fact that when argentiferous copper pyrites is roasted, the copper and iron sulphides are converted into insoluble oxides, whilst the silver is converted into a soluble sulphate which is dissolved out by lixiviating the roasted ore with hot water, the silver being readily precipitated from this solution in the metallic state." It is certain that if an observant, chemically trained student visited a silver extraction works, and possessed sufficient analytical skill to enable him to secure evidence as to the changes that occur, he would find a set of facts which his training had not enabled him to predict, and he would establish the existence of a set of reactions to the nature of which his chemical reading had hardly given him a clue. The process to be considered is a simple one, but it is typical, and applies to a large proportion of the 7,000,000 ounces of silver annually obtained in the world from cupriferous compounds. He would be confronted with a ton or more of finely divided material spread in a thin layer over the bed of a reverberatory furnace. Suppose the material is what is known as a complex "regulus" as imported into Swansea or produced at Frieberg, to which are added rich native sulphides. The mixture then consists of sulphides mainly of iron and copper, with some sulphide of lead, and contains fifty or sixty ounces of silver to the ton, and a few grains of gold. It may also contain small quantities of arsenic and antimony as arsenides, antimonides, and sulpho-salts, usually with copper as a base. The temperature of the furnace in which the operation is to be performed is gradually raised, the atmosphere being an oxidising one. The first effect of the elevation of the temperature is to distil off sulphur, reducing the sulphides to a lower stage of sulphurisation. This sulphur burns in the furnace atmosphere to sulphurous anhydride (SO,), and coming in contact with the material undergoing oxidation is converted into sulphuric anhydride. (SO). It should be noted that the material of the brickwork does not intervene in the reactions, except by its presence as a hot porous mass, but its influence is, nevertheless, considerable. The roasting of these sulphides presents a good case for the study of chemical equilibrium. As soon as the sulphurous anhydride reaches a certain tension the oxidation of the sulphide is arrested, even though an excess of oxygen be |