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STEP by step modern science penetrates deeper and deeper into the intimate structure of physical bodies, and the new step which we have now to record is the progress made in our knowledge of the inner molecular structure of solids. It may seem strange, of course, that physicists should have found difficulties in interpreting the structure of so commonplace a thing as a stone, or a block of lead, copper, or silver. But it must be remembered that what we want to know about the solids is not the arrangement of their rougher particles (that much' is learned easily enough with the aid of the microscope); we want to penetrate far beyond the utmost limits of microscopical vision; to know how the molecules, which are minute as to defy the powers of our best microscopes, are arranged ; how they are locked together; in how far they are free in their movements, and what sort of movements they perform ; what is, in a word, the inner molecular life of a seemingly inert block of metal. Such a question could not be answered directly, and the problem had to be attacked in all sorts of roundabout ways. Attempts to solve it were made, accordingly, in more directions than one, and in these 'attempts physicists grasped first the molecular structure of gases; then it took them years to extend their knowledge to liquids; and it is only now that some definite results have been arrived at as regards solids through the combined efforts of a great number of chemists, physicists, and metallurgists."
· For penetrating into this vast domain no better guide could be found for the general reader than Prof. W. C. Roberts-Austen's Introduction to the Study of Metallurgy (1st edition in 1891 ; 3rd edition in 1895), which contains, besides excellent reviews of the whole domain, copious bibliographical indications. C. W. Roberts. Austen's lectures before the Royal Society, the Royal Institution, and the British Association, all published in Nature, deserve the same mention :- On the Hardening and Tempering of Steel' (1889, Nature, vol. xli. pp. 11 and 32); • Metals at High Temperatures' (1892, vol. xlv. p. 534); • The Rarer Metals and their Alloys' (1895, vol. lii. p. 14 and 39); The Diffusion of Metals' (1896, vol, liv, p. 55). Also his three · Reports to the Alloys Research Committee of the Institution of Mechanical Engineers’ in 1891, 1893, and 1895, and the subsequent discussions. For a general review of the alloys, considered as solutions of metals in metals, the second volume of Ostwald's Allgemeine Chemie (Leipzig, 1893 ; English translation in 1894) is the
We conceive gases as consisting of an immense number of molecules which dash in all directions, continually meeting each other in their rapid movements, and consequently changing their courses, and continually endeavouring to escape into space. The more we heat a gas, the more agitated become the movements of its molecules, and the greater become their velocities. To raise the temperature of a gas simply means, in fact, to increase the velocity of the movements of its molecules. These molecules, as they dash in all possible directions, bombard the walls of the vessels which a gas is enclosed in, and take advantage of every issue to escape through it; and although they are extremely small in size, their numbers are so great and their movements are so rapid that they even break the walls of the strongest receptacles. When they bombard the piston of a steam-engine, they push it with such a force that it can move heavy masses or set in motion a heavy railway train at a considerable speed.
Such a conception of the structure of gases (the kinetic theory of gases ') was first propounded as an hypothesis only; but it so remarkably well corresponds to realities, it gives us so full an explanation of all phenomena relative to gases, and it permits us to foretell so many phenomena, that it may already be considered as a well-established theory. We measure the velocities of the molecules, and even attempt to count the numbers of their impacts as they dash against each other; we have an approximate idea of the sizes of some of them-sieves having been imagined which let the smaller molecules pass but intercept the bigger ones; and, maybe, Messrs. H. Picton and S. E. Linder, in their researches into solutions of sulphide salts, have even seen under the microscope how some bigger molecules aggregate into particles.
So far the inner structure of gases is known ; but as regards the inner structure of liquids our views are much less definite. We know that liquids are also composed of molecules, or of groups of molecules (particles), which very easily glide upon and past each other. Gravitation makes them glide so as to fill up every nook of a vessel, flow through its apertures, and produce a horizontal surface on the top of the liquid; and if we heat any part of the liquid, currents and eddies are immediately produced-particles gliding past each other in various directions. But until lately, if the physicist was asked whether, apart from these movements due to extraneous causes, the liquid molecules have not their own movements, like the gaseous ones, he hesitated to give a definite reply. These doubts, however, have been removed within the last twenty years. By this time there is not one single gas left which would not have been brought into a liquid state. Every gas, if we sufficiently compress and cool it—that is, bring its molecules into closer contact and reduce the speed of their oscillations—is transformed into a liquid, and, before being liquefied, passes through an intermediate, 'critical state, in which it combines the properties of a liquid with those of a gas. Moreover, it has lately been proved that mechanical laws which hold good for gases are fully applicable to liquid solutions, as if they really contained gaseous molecules, and we are bound to recognise that there is no substantial difference between the inner structure of a gas and a liquid—the difference between the liquid and the gaseous states of matter being only one of degree in the relative freedom, mobility, and speed of molecules, and perhaps in the size of the particles.
sarest guide. The general parts of the papers of W. Spring and Van der Mensbrugghe (mentioned hereafter) are very suggestive. Otto Graham's Collected Papers’are a rich mine of suggestive information which need no recommendation. Behrens's book, Das mikroskopische Gefüge der Metalle und Legierungen (Leipzig, 1894), can also be warmly recommended. Special researches are mentioned further down.
? No human hand could make such a sieve; but Warburg and Tegetmeier have imagined a means of locking the molecules of sodium out of a pan of glass. Through the minute channels thus obtained, molecules of sodium make their passage, as also the still smaller molecules of lithium, while the bigger ones of potassium are intercepted.
Can we not, then, extend our generalisation, and say that the difference between a solid and a liquid is not greater than between a liquid and a gas ? For simplicity's sake, let us take a block of pure metal. Like all other physical bodies, it consists of atoms grouped into molecules and of molecules grouped into particles, and it is known that these last cannot be solidly locked to each other, because each rise of temperature increases the volume of the metallic block and every blow makes it emit a sound. The molecules must consequently have a certain mobility, since they can enter into sonorous and heat vibrations. But to what extent are they free ?
Do they not enjoy-some of them, at least—such a freedom of movement that they can travel, as they do in liquids and gases, between other molecules, from one part of the solid to another ? Do they not maintain in the solid state some of the features which characterise their movements in both the liquid and gaseous states ? This is, in fact, the conclusion which science is brought to by recent investigations. As will be seen from the following facts, it becomes more and more apparent that a solid piece of metal is by no means an inert body; that it also has its inner life; that its molecules are not dead specks of matter, and that they never cease to move about, to change places, to enter into new and varied combinations.
It was especially through the study of alloys, for both industrial and scientific purposes, that modern science was brought to the above
• This stage bas been treated at some length in a preceding article, Nineteenth Century, April 1894.
* Ibid. August 1892.
views; and therefore we are bound to make an incursion into that vast domain. An alloy is not a simple mixture of two metals; far from that. It stands midway between the physical mixture and the chemical compound, and combines the characteristics of both. If we take, for instance, some molten lead and throw into it a piece of tin, or add molten zinc to molten copper in order to obtain brass, or mix molten copper and silver in order to make silver coins, we do not obtain simple mixtures of lead and tin, copper and zinc, or silver and copper. We produce quite new metals, totally different from their component parts; not true chemical compounds, and yet not mixtures. The alloy has a different colour, a different hardness or brittleness; it offers a quite different resistance to the passage of electricity; and it requires, for fusion, a temperature which is generally much lower than the temperatures of fusion of its two or three component metals. We take, for instance, 118 parts of tin, 206 parts of lead, and 208 parts of bismuth, as finely divided as possible, mix them as rapidly as we can with 1,600 parts of mercury, and we obtain a freezing mixture of so low a temperature (14° Fahr.) that water can be frozen in it. Or, we take 15 parts of bismuth, 8 parts of lead, 4 parts of tin, and 3 parts of cadmium, and we obtain a metal which fuses in boiling water (at 209° Fahr.), although the most fusible of the four metals, i.e. tin, requires a temperature of, at least, 446 degrees to be melted, and cadmium does not fuse before the heat has reached 576 degrees.”
Nay, all the physical properties, and the very aspect of a metal, can be changed by merely adding to it a minute portion of some other metal. Thus, the very aspect of pure bismuth can be so changed by adding to it doth part of tellurium (a rare metal, found in small quantities in combination with gold, silver, etc.), that, as Roberts-Austen remarks, one could readily take it, on mere inspection, for a totally distinct elementary body. The addition of twenty-two per cent. of aluminium makes gold assume a beautiful purple colour ; but gold can also be made to assume a greenish colour, and its strength can be doubled, by adding to it both part of one of the rare metals, zirconium; while the addition of another rare metal, thallium, in the same minute proportion, would halve the strength of gold. Nay, we may obtain gold which will soften in the flame of a candle by adding to it toth part of silicon. As to copper, it is known that its electric conductivity is so rapidly diminished by the presence of the slightest impurities of other metals, that if the copper of which a cable is made contained only Tooth part of bismuth, this impurity would be fatal to the commercial success of the cable.' 6
$ I follow in these illustrations Roberts-Austen's Introduction to the Study of Metallurgy. • Ibid., p. 76.
As to the immense variety of different sorts of metals which are obtained by adding small quantities of carbon to iron in the fabrication of steel, or by introducing very small quantities of manganese or chromium into steel, it would be simply impossible to enter into the subject in this place, so vast and interesting is it. Suffice it to say that, beginning with pure iron, which can be had as soft and pliable as copper, and ending with steel which is hard enough to cut glass, or with those chrome-steel shells which pierce nine-inch armour plates, backed by eight feet of solid oak, without their points being deformed, there are all possible gradations of iron alloys. And it becomes more and more apparent, from the work of Osmond, Behrens, and many others, that steel contains not only five different constituents—partly chemical compounds of iron and carbon, and partly solutions of carbon in iron alloyed in different proportions--but also iron and carbon appearing in different molecular groupings of their atoms (allotropic forms), microscopic diamonds inclusive. A block of an alloy is thus quite a world, almost as complicated as an organic cell.
Besides, a close resemblance has been proved to exist between alloys, so long as they remain molten, and solutions of salts in water and other solvents. When a piece of tin is dissolved in molten lead, , or two molten metals are mixed together, the same complicated physical and chemical phenomena are produced as in dissolving a lump of salt in water or mixing alcohol with water. The physical properties of the metal used as a solvent are entirely altered as the molecules of the dissolved metal travel, as if they were in a gaseous state, amidst its own molecules. Some of them are dissociated at the same time, and new chemical compounds of an unstable nature are formed, only to be destroyed and reconstituted again. In a word, all laws based on the assumption of a nearly gaseous mobility of molecules and atoms, which have been found to be applicable to solutions of salts in water, can be fully applied to molten alloys as well.o And the question necessarily arises : whether the mobility of molecules
? Mr. Hadfield's paper, read before the Iron and Steel Institute on the 21st of September, 1892 (Nature, vol. xlvi. p. 526).
8 Roberts-Austen has summed up some recent French works on this subject in a paper contributed to Nature (1895, vol. lii. p. 367). See also his earlier lecture on steel, incorporated in his Introduction to Metallurgy. Diamonds have been extracted from common, very hard steel by Rossel (Comptes Rendus, 13 juillet, 1896, p. 113).
9 Hancock and Neville have proved by their admirable series of researches (since 1889) that all laws which have been established for solutions by Ostwald, Van't Hoff, and Arrhenius are applicable to alloys. The freezing-point' is lowered in alloys as well, in proportion to the number of molecules of the dissolved metal added to the solvent (Tamman, Ramsay, Hancock, and Neville). At the same time, many perfectly homogeneous alloys, just as homogeneous as certain solutions, have been obtained (see also the extensive researches on ternary alloys by Dr. Alder Wright in the Proceedings of the Royal Society since 1889, and in the chapter he has contributed to the third edition of Roberts-Austen's Introduction). The number of chemical compounds formed by two metals in alloys, in analogy with the chemical compounds formed in