September 9, 1920] 



NATURE 



61 



laws, when applied to aqueous solutions of electro- 

 lytes, could be explained by the assumption, first made 

 by Arrhenius, that these iatler in solution are partly 

 dissociated into their ions. The result of all this 

 work was to establish a general theory applicable to 

 all solutions which has been widespread in its 

 applications. From this time the study of alloys began 

 to make rapid progress. 



The experiments of Laurie, Tannman, and Neville 

 and myself in 1888 and 1889 helped to establish the 

 similarity between the behaviour of metallic solutions 

 or alloys and that of aqueous and other solutions of 

 organic compounds in organic solvents. That our ex- 

 periments were correct seemed probable from the 

 agreement between the observed depression of the 

 freezing point and the value calculated from van't 

 Hoff's formula for the case of those few metals the 

 latent heats of fusion of which had been determined 

 with any approach to accuracy. 



Our experiments, afterwards extended to other 

 solvents, led to the conclusion that in the case of most 

 metals dissolved in tin the molecular weight is identical 

 with the atomic weight ; in other words, that the 

 metals in solution are monatomic. This conclusion, 

 however, involves certain assumptions. Sir William 

 Ramsay's experiments on the lowering of the vapour 

 pressure of certain amalgams point to a similar con- 

 clusion. 



So far our work had been carried out with mercury 

 thermometers, standardised against a platinum resist- 

 ance pyrometer, but it was evident that, if it was to 

 be continued, we must have some method of extending 

 our experiments to alloys which freeze at high tem- 

 peratures. The thermo-couple was not at this stage a 

 trustworthy instrument ; fortunately, however, Cal- 

 lendar and Griffiths had brought to great perfection 

 the electrical resistance pyrometer (Phil. Trans., A, 

 1887 and 1801). Dr. E. H. Griffiths kindly came to 

 our aid, and with his help we installed a complete 

 electrical resistance set. ,\s at this time the freezing 

 points of pure substances above 300° were not known 

 with any degree of accuracy, we began by making 

 these measurements : — 



lahU of Freezing Points. 



and !.« 



with the exception of silver and gold, these metals 

 were the purest obtainable in commerce. 



During the period that the above work on non- 

 ferrous alloys was being done, great progress was 

 being made in the studv of iron and steel by Osmond 

 and L<> Chatclier. In i8<x> the Institution of 

 Mechanical Engineers formed an Alloys Research 

 Committee. This committee invited Prof^ (afterwards 

 Sir Willi.im) Roberts-Ausfen to undertake research 

 work for it. The results of his investigations are con- 

 tained in a series of five vnlunble reports extending 



NO. 2654, VOL. 106] 



from 1891 to 1899, published in the Journal of the 

 institution. The fifth is of especial importance, because, 

 besides a description of the thermal effects produced 

 by carbon, which he carefully plotted and photo- 

 graphed, he described the microscopical appearance of 

 the various constituents of iron. The materials of 

 this report, together with the work of Osmond and 

 others on steel and iron, provided much of the material 

 on which Prof. Bakhuis Roozeboom founded the 

 iron-carbon equilibrium diagram. Reference shou'd 

 also be made to the very valuable paper by Stansfield 

 on the present position of the solution theory of 

 carbonised iron (Journ. Iron and Steel Inst., vol." xi., 

 1900, p. 317). It may be said of this fifth report, 

 and of the two papers just referred to, that they form 

 the most important contribution to the study of iron 

 and steel that has eser been published. .Mthough 

 the diagram for the equilibrium of iron and carbon 

 does not represent the whole of the facts, it affords 

 the most important clue to these alloys, and un- 

 doubtedly forms the basis of most of the modern 

 practice of steel manufacture. Many workers, both 

 at home and abroad, were now actively engaged in 

 metallurgical work — Stead, Osmond, Le Chatelier, 

 Arnold, Hadfield, Carpenter, Ewing, Rosenhain, 

 and others too numerous to mention. 



In 1897 Neville and I determined the complete 

 freezing-point curve of the copper-tin alloys, con- 

 firming and extending the work of Roberts-Austen, 

 Stansfield, and Le Chatelier; but the real meaning 

 of the curve remained as much of a mystery as ever. 

 Early in 1900 Sir G. Stokes suggested to us that we 

 should make a microscopic examination of a few 

 bronzes as an aid to the interpretation of the sin- 

 gularities, of the freezing-point curve. .An account of 

 this work, which occupied us for more than two 

 vears, was published as the Bakerian lecture of the 

 Royal Society in February, 1903. Whilst preparing 

 a number of copper-tin alloys of known composition 

 we were struck by the fact that the crystalline pat- 

 tern which developed on the free surface of the slowly 

 cooled alloys was entirely unlike the structure 

 developed by polishing and etching sections cut from 

 the interior; it therefore appeared probable that 

 changes were going on within the alloys as they 

 cooled. In the hope that, as Sorby had shown in the 

 case of steel, we could stereotype or fix the change 

 by sudden cooling, we melted small ingots of the 

 copper-tin alloys and slowly cooled them to selected 

 temperatures and then suddenly chilled them in 

 water. The results of this treatment were com- 

 municated to the Royal Society and published in the 

 Proceedings of February, 190 1. 



To apply this method to a selected alloy we first 

 determined its cooling curve by means of an auto- 

 matic recorder, the curve usually showing several 

 halts or steps in it. .The temperature of the highest 

 of the.se steps corresponded with a point on the 

 liquidus, «.e. when solid first separated out from the 

 molten mass. To ascertain what occurred at the later 

 halts, ingots of the melted alloy were slowly cooled 

 to within a few degrees above and below the halt 

 and then chilled. 



The method of chilling also enabled us to fix, with 

 some degree of accuracy, the position of points on 

 the solidus. if an alloy, chilled when it is partlv 

 solid and partly liquid, is polished and etched, it will 

 be seen to consist of large primary combs enil>edded 

 in a m.itrix consisting of mother-liquor, in which arc 

 disseminated numerous small combs, which we called 

 "chilled primary." By repeating the process at suc- 

 cessively lower and tower temperatures we obtained 

 a point at which the rhille<I primary no longer 

 formed, i.e. the upper limit of the solidu*. 



