field of investigation, alloys were last to receive serious attention or, at least, to yield fruitful results. The physical methods successfully applied to bodies of other kinds failed utterly to bring to light a rational explanation of the molecular conditions existing in alloys. The solution of these problems required new methods and new apparatus. These have been slowly forthcoming, and the past decade has been most fruitful of results, and problems of inestimable practical importance and extreme difficulty have been solved.

MODERN ALLOYS RESEARCH.

With a few exceptions, the great leaders in this work are a contemporary school of English and French metallurgists and molecular physicists. In the pages that follow, we aim to give a résumé of their achievements, and to outline our present knowledge of the nature and constitution of binary alloys. There are three important factors which account for the recent progress in alloys research. (1) increased knowledge upon the subject of solutions. (2) the development of the science of matallography. (3) improvements in pyrometry. To these we might add a fourth, namely, the official recognition of the importance of investigation along these lines which, over ten years ago, led the Institution of Mechanical Engineers of Great Britain to appoint an "Alloys Research Committee," which receives financial aid from the Institution; and the Société d'Encouragement pour l'Industrie Nationale to create a 'Commission des Alliages," for whose researches nearly all the railway companies of France and many metallurgical concerns contributed generously.

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For advancing our knowledge of solutions we must give credit to the great physical chemists of Germany and Holland -Ostwald, Nernst, Van't Hoff, Roozeboom, and others. The microscopic study of metals seems to have originated with Dr. Sorby, an Englishman, who wrote upon the microstructure of meteoric iron as early as 1864. Only within the past ten or fifteen years has this method of investigation obtained just recognition among scientists as a valu

able aid to metallurgical research, and not even yet has it obtained proper recognition among manufacturers. Recent progress in metallography is most nearly associated with the names of Stead, Martens, Osmond, Le Chatelier, Charpy, Roberts-Austen, Howe and Sauveur.

In the purely scientific study of the cooling curves of metals and alloys, two systems of pyrometry have given valuable results. The Siemens electrical resistance pyrometer, perfected by Callendar and Griffiths, is a reliable instrument, and, within certain limits, gives wonderfully accurate readings. For use both in the laboratory and in the factory it does not seem so satisfactory or so popular as the Le Chatelier thermo-electric couple. Not least among the advantages of the platinum, platinum-rhodium couple are, its accuracy at high temperatures, its ability to be used with small masses of heated substances, and, indirectly, its adaptability to photo-autographic recording of the temperatures indicated by such an instrument as the one invented by Sir William Roberts-Austen.

CRYSTALLINE STRUCTURE OF PURE METALS.

Before considering the constitution of alloys, it is well to know something of the general properties of metals. All solid metals are crystalline, although in microsection the individual crystals or crystalline grains may not present a simple geometrical outline. Professor Ewing, of Cambridge, has pointed out in this regard that the essential point is that the particles composing the mass of a crystal lie in one direction, i. e., have the same plane of orientation, He also explains crystallization in a metal in this way: The formation of crystals must be assumed to start simultaneously at many points. The crystals grow until they touch one another; thereafter their symmetrical growth is impeded, but it is not necessary that the corresponding axes of any two of them be lying in exactly the same plane. In the case of commercial metals, more or less impure, the impurities are cast out by the growing crystals, and being, in fact, alloys of the admixed impurities with a little of the principal metal, have a lower melting point than

the pure metal. Hence they solidify last and form an investing cement which holds together the primary crystals.

THE EFFECTS OF STRAIN IN PURE METALS.

Within the elastic limit no change is noticeable upon the polished surface of a metal or alloy when the same is under stress. When, however, the plastic stage is reached, there will appear dark lines more or less perpendicular to the direction of stress. This, Professor Ewing says, is not due to fissuring, for by changing the direction of the rays which illuminate the specimen under observation from vertical to oblique, it will be seen that the erstwhile dark lines appear light, while the background has changed from light to dark. Fissures would not reflect light in any case; the permanent elongation is due to slipping of the components of the crystals past one another. The slip lines need not necessarily lie in one plane. As many as four sets of parallel slip lines have been noticed by Professor Ewing.

My colleague, Mr. William Campbell, studied this phenomenon independently at about the time that Professor Ewing was making these interesting observations. Mr. Campbell has very kindly allowed me to exhibit here for the first time some of his results. Mr. Campbell worked with tin, while Professor Ewing experimented chiefly with lead. It is very interesting to notice how completely their results agree.

Fig. 1 shows the growth of crystals in tin. Two samples were rolled and then annealed for ten days upon a hot plate, below 200° C. The annealed specimens were etched and show in a very striking manner, not only the growth of crystals, but also the different planes of orientation. The figure shows these crystals slightly reduced from their natural size. Figs. 2, 3 and 4 show respectively the same specimen of hammered tin; 2 is the original, etched in HC1, magnified 33 diameters, oblique illumination. On standing eighteen months the appearance changed to that shown in 3, also magnified by 33 diameters, oblique; and 4 shows the effect of annealing 3 for ten days on the hot plate, but it is only magnified by 16 diameters. In Fig. 5 we have

a microsection of No. 4, which shows wonderfully well lines due to strain which Mr. Campbell thinks was set up by the rapid growth. The fact that in different crystals the lines are parallel tends to confirm this view, the strain not having been manifested in a single direction. When such is the condition due to stress, a condition such as is illustrated in Fig. 6 is set up, in which the general direction of the slip lines is normal to the direction of stress.

FREEZING-POINT OF METALS.

With water and many other liquids it is quite possible to lower the temperature to a considerable amount below their freezing-points without any separation of solid taking place. On the other hand, there is no known substance which can be heated above its melting-point without becoming liquid. When a liquid which exhibits this phenomenon of surfusion is thus cooled, and solid matter begins to separate, the temperature rises rapidly to the true freezing-point and remains constant until the whole mass is solid. The same is true of pure metals. Sir William Roberts-Austen cites an instance in which tin was cooled 20° C. below its freezing-point without solidifying. The author has frequently observed the same thing in tin and other metals, but to a less degree. In working with metals we take the first point at which solids separate as the freezing-point. For practical purposes we may consider the freezing-point and the melting-point as identical, but the latter is very difficult to determine, while the former is not. The phenomenon of surfusion need not obscure the determination of the correct point, particularly if we obtain an autographic record of the cooling curve. If we plot a typical cooling curve of a pure metal, using temperature and time as the coördinates, we obtain a curve like AB (Fig. 7) n which the angles are very distinct, i. e., the temperature remains constant during the whole of solidification. If surfusion takes place-and it is more common in pure metals than in alloys-the curve A' B' represents what happens. The temperature falls below the real freezing-point and then rises abruptly to that point and remains constant as