June 23, 1904] 



NA TURE 



187 



This was found to make less differeiue to the final result 

 than that caused by observing the different oppositions. 



The results obtained for the position of the pole of Mars 

 are as follows : — 



R.A. Dec. 



Position upon the earth's equator... 315° 32' 54° 51' 

 Intersection of the Martian equator 



and Martian ecliptic ... ... 85° 56' 24° 32' 



Inclination of Martian equator to 



Martian ecliptic ... ... ... — 22° 55' 



THE STRUCTURE OF METALS.' 

 T^HE subject of the lecture was the structure of metals, 

 mainly as revealed by the microscope. The first serious 

 application of the microscope to the study of metallic struc- 

 ture was made in 1S64 by Dr. H. Sorby, of .Sheffield, but 

 the lead then given was not followed for nearly a quarter 

 <if a century. In the last fifteen years or so, however, it 

 had been taken up with the greatest zeal and success, 

 nowhere more than in Dr. Sorby 's own town. There and 

 elsewhere, in France, Germany, and .\merica, as well as 

 at home, a band of enthusiastic workers had been engaged 

 in creating what might be described as a novel branch of 

 physical science, as interesting on the physical side as it 

 was important in its practical aspect. In this work Cam- 

 bridge had done its share. The lecturer referred especially 

 to work done in the engineering laboratory by Rosenhain, 

 Humfrey, and other of his own former lesearch students. 

 <md to the admirable investigation of alloys carried out by 

 Neville and Heycock in the laboratory of Sidney Sussex 

 <"ollege. 



It w'as only possible to give in a single lecture a very 

 brief account of part of this work. Photography had lent 

 its powerful aid in recording what the microscope made 

 visible. By means of lantern slides showing micro- 

 photographs of polished and etched metallic surfaces, the 

 lecturer proceeded to exhibit the characteristic structure of 

 ;i pure or nearly pure metal, where the whole mass is made 

 .up of irregular grains with well marked boundaries more or 

 less polygonal in form. The grains could be distinguished 

 from one another not only by the presence of the boundaries, 

 but by differences of texture which were especially con- 

 spicuous under oblicjue lighting. Each grain was a true 

 crystal made up of similarly oriented particles in a perfectlv 

 regular tactical arrangement, such as might be exemplified 

 by imagining it to be built up of minute brickbats all of 

 the same form and size. When a polished surface was 

 etched the facets of the elementary brickbats were exposed. 

 ■and the manner in which these reflected the light into or 

 .away from the microscope determined the appearance which 

 the grain presented under oblique illumination. A slight 

 change in the direction of the incident light would greatly 

 .affect the "brightness of the grain, making it shine out or 

 grow dull, but over each grain there was a uniform degree 

 •of brightness due to the uniformity of its tactical formation. 

 Each grain had grown as a crystal, starting from a chance 

 nucleus, and the boundaries were determined by the casual 

 interference of grain w'ith grain in the process of growth. 

 In general, the growth was at first dendritic, skeleton forms 

 shooting out until they met similar growths in neighbour- 

 ing grains, and the interstices of the skeleton were filled in 

 later. In some metals the grains were products of 

 crystallisation from the liquid state; in others, notably in 

 iron, n re-crystallisation took place long after the metal had 

 solidified, and in such cases the grains, as we knew them 

 xmder ordinary conditions of temperature, w'ere the result 

 •of an internal re-arrangement which took place while the 

 metal was solid. In such cases they were characterised by 

 less regular boundaries, and there was evidence of more 

 intimate interlocking between grain and grain. The struc- 

 ture might be fine or gross; in specially pure metals, and 

 under specially slow conditions of cooling, it was apt to 

 become specially gross. An instance was exhibited of a 

 piece of lead of exceptional purity allowed to solidify by 

 very slow cooling, in which the grains were so large as to 

 lie visible to the audience without magnification. Their 

 .-qjpearani-e under oblique lighting was projected on the 



1 Abstract of the Rede lecture delivered before the University of Cam- 

 bridge, lune ti. By J. A. Ewing, LL.D., F.R.S., Hon. Fellow of King's 

 •College,' Director of Naval Education. 



NO. 1808, VOL 70] 



screen, and by tilting the block of lead the striking changes 

 of brightness due to change in the incidence of the light 

 w^ere e.xhibited. Other evidences of the crystalline character 

 of the grains were referred to, namely, the pits and 

 geometrical forms developed on the surface by etching, and 

 the geometrical forms assumed by very minute bubbles of 

 gas or air imprisoned in the process of solidification. 



Coming next to the consideration of effects of stress, the 

 lecturer described the experiments bv which, in conjunction 

 with .vir. Rosenhain, he had demonstrated that the plastic 

 yielding of metals when severely strained is due to a multi- 

 tude of slips occurring along cleavage planes in the several 

 grains of which the metal is a conglomerate. The appear- 

 ance of " slip-lines " in various metals was shown, and the 

 character of the lines was discussed. .\s Rosenhain had 

 recently pointed out, the slip-lines were comparativelv 

 straight in grains formed by solidification from the solid 

 (as, for example, in cast lead, silver, and gold), but were 

 broken up into steps which gave them the appearance of 

 being curved in metal which had undergone re-crystallisation 

 while in the solid state. This was ascribed to the more 

 intimate interlocking of the grains in the latter case. That 

 the slips showed themselves by steps or sudden slight 

 changes in the level of the surface was clearly demon- 

 strated when the slip-lines were examined under oblique 

 light. All the parallel slips on a given grain would then 

 flash out simultaneously w'hen the direction of the incident 

 light suited the particular slope of the planes in which 

 the slips had taken place. The form of slips in twin 

 structures was exhibited, and also in an example (due to 

 Humfrey) of lead with a structure so gross that the relation 

 of the slips to the geometry of the grain could be readily 

 traced. 



.\ question of immense practical interest was the 

 " fatigue " which metals underwent when exposed to many 

 repetitions of a straining action. The microscope threw 

 valuable light on this by showing how, under repetition of 

 pulls or pushes or bendings, a piece began to give way, 

 first by slips appearing on isolated grains, and then by 

 some of these slips gradually developing into cracks. 

 Instances were cited from a joint research by the lecturer 

 and Mr. Huinfrey. Mr. Rogers, who had pursued this 

 subject with much zeal, had recently found that breakdown 

 by fatigue was much more liable to occur in steel which 

 had been thermally treated in such a manner as to develop 

 a comparatively large structure than in the same steel 

 when the treatment was such as to make the structure 

 normally small. 



Going on to speak of alloys, the lecturer described shortly 

 the various ways in which two constituents might combine, 

 or rather act together, in the composition of a binary alloy. 

 In the liquid state each dissolved in the other, in the solid 

 state one might remain wholly or in part dissolved in the 

 other, foriTiing what was called a solid solution. Thus 

 with two constituents A and B, if A were present in small 

 quantity only it might be found wholly contained as a 

 solid solution in B. More generally, however, a solid 

 solution would crystallise out first, leaving a mother liquor 

 richer in A, which, by throw'ing down more and more solid 

 solution, finally reached the proportion of the " eutectic " 

 alloy, and then solidified as a eutectic mixture, showing 

 under the microscope the zebra-like marking which 

 characterised eutectic alloys. This process w'as explained 

 by means of freezing-point curves, and was exemplified by 

 a beautiful series of photographs taken by Mr. Stead, 

 showing alloys of various proportional composition in which 

 iron and phosphide of iron were the two constituents. 

 When very little phosphorus was present, the whole solidified 

 as a solid solution showing grains undistinguishable in 

 general appearance from those of a pure metal. With a 

 little more phosphorus the solid still consisted mainly of 

 large grains, but the interstices (or in one case the inner 

 parts of a dendritic skeleton) showed traces of the eutectic. 

 which was the last part to solidify. With more phosphorus 

 still the solid solution showed itself as incomplete skeleton 

 grains interspersed with large quantities of eutectic. With 

 more still the eutectic proportion was reached, and the 

 whole solidified as a eutectic mixture, showing zebra mark- 

 ings all over the surface. With more still — that is to say, 

 with an excess of phosphide — crystals of phosphide were first 



