

ON THE CRYSTALLINE BTRPOTUBE OK METALS. 355 



beautiful surface was obtained. In some cases ixdished steel was used as the smooth 

 object against wbich the metal was pressed. 



The jxilished surfaces were, in general, slightly etched before microscopic exami- 

 n.it ion, in most instances by dilute nitric acid. Frequently, however, no etching was 

 n sorted to, especially in ol>servations dealing with the effects of strain. It has l>een 

 pointed out IA ('iiAiM'Y that straining, by the relative displacement which it brings 

 alxmt among the crystalline grains, serves to reveal the structure. < >ur olwervations 

 conn* rni this, and in some cases we found that a lietter investigation of the effects of 

 strain could lx- made when the surface was not etched. 



Most of the microscopic ol nervations were made with ZKISS' apochromatic lenses, a 

 2-millim. homogeneous immersion lens of 1'40 aperture being employed for high 

 power work, with oompentttmg eye-pieces magnifying from 4 to 18 times, giving in 

 direct vision a total magnification up to 3000 diameters. "Vertical" illumination 

 \\.is ^friierallv u-ed. tin- ..lij.-.-t i\ i- v,.|\'n,-_;- M MndenMr, I'Ut 111 v,,in,- OMM '!" 

 specimens were examined under oblique liglit. Photographic records of the most 

 interesting features were obtained, some of which are reproduced in this paper. The 

 source of liglit was an arc lamp, the beam from which was condensed on the illumi 

 n.-itor through a " Gifford " screen which allowed only a very limited portion of the 

 sped ruin to pass. Most of the high jxiwer j)hotogra])lis were taken with a magnification 

 of 1000 diameters ; in a few instances it was 4000 diameters or more. 



It is \\ell known that when the polished surface of a metal, such as gold or iron, is 

 lightlv etched, and is then examined by means of normally reflected (" vertical ") light, 

 the surface apj>ears divided up into a numl>er of areas separated by more or less 

 polygonal. Iwii ndaries. These areas are the sections of the crystalline grains which 

 constitute the mass of the metal; the Ixmndaries between them have been made 

 evident by the differential action of the acid which has produced differences of level 

 by attacking one grain more energetically than its neighbour. Fig. 2 (Plate 15) 

 illustrates this ap}>earance in ordinary iron. There the black patches are due to the 

 presence of slag, and the black lines forming the boundaries are due to differences of 

 level l>etween the grains. Each of the short sloping surfaces which connects one 

 i in with another appears black localise it does not reflect the normally incident 

 light Iwiek into the tube. 



It is also well known that a further differential action of the acid is to reveal a 

 difference of texture lietween the grains. This is visible in fig. 2, but is much more 

 pronounced when the surface is examined under oblique light. When the light is 

 uni-directional or n>arlv so. the various grains differ very much in brightness and 

 colour some are almost black, while others shine out brightly; if, however, the 

 incidence >(' the light or the orientation of the specimen l>e changed, other grains 

 shine out strongly, while those previously bright l>ecome dark. This effect was first 

 observed in gold by ARNOLD (' Engineering,' February 7, 1896), who accounted for it 

 liv considering that each crystalline grain is built up of a very large numl>er of what 



:.' /. 1 



