3515 PROFESSOR .T. A. EWTXC A\H MR. AY. ROSENHAIN 



he (following ANDREWS) calls sccuiul;iry crystals. \\-lm-li have different orientations in 

 different grains. Substantially the same view is expressed in other terms by OSMOND 

 and ROBERTS- AUSTEN ('Phil. Trans.,' A, vol. 187, pp. 424-5), who speak of little 

 crystals, "the general orientation of which remains constant in the area of each 

 grain." Many beautiful illustrations of the same effect in iron have l)een given 1 IN- 

 STEAD, along with a clear discussion of the cause to which this appearance is to be 

 ascribed ('Journal of the Iron and Steel Institute,' 1898). A striking instance is 

 shown in his photographs of steel containing about 4 per cent, of silicon. The 

 fractured ingot of this material exhibits large crystals, and by deeply etching a 

 polished surface he has obtained a fine development of the regiilarly oriented elements 

 of which the crystalline grains are built up, on a scale so large as to l>e clear with 

 little or no magnification. 



These observations have made it plain that each of the grains which appear on the 

 polished and etched surface of a metal is simply a crystal, the growth of which has 

 been arrested by its meeting with neighbouring grains. The irregular boundaries of 

 the grains are determined by these meetings. We may imagine the grains to grow 

 from as many centres or nuclei as there are grains. Each grain is built up ot 

 similai'ly oriented parts, but the orientation changes from grain to grain. Etching a 

 polished surface develops a multitude of facets which have the same orientation over 

 the surface of any one grain, but different orientations in different grains. Seen 

 under oblique illumination these facets show themselves to be similarly oriented in 

 each grain by the uniform manner in which the grain reflects light, and by the 

 disappearance of brightness over the whole surface of the grain when the incidence 

 of the light is changed. The mass of the grain consists of similarly oriented 

 elements ; as to the size of the elements no assumption need lie made. The facets 

 which are developed by etching do not, in general, appear of constant size ; it is to 

 the constancy of their orientation that the effect is due. 



A striking illustration of how a metal crystallises by the simultaneous building up 

 of groups of elements from a number of centres, the elements in each group being 

 similarly oriented, while the orientation varies from group to group, may be seen in a 

 cake of solidifying bismuth from which .the still molten metal has been poured away. 

 The operation then goes on upon a scale so large that no magnification is required to 

 make it apparent. Fig. 3 is a photograph of a specimen of bismuth in the Cambridge 

 University Museum of Mineralogy, for the loan of which the authors are indebted t 

 Professor LEWIS and Mr. A. HUTCHINSON. The scale of the photograph is only alxnit 

 two-fifths natural size, but it shows well how the cake is made up of crystalline 

 grains, each composed of elements with a definite orientation, the boundaries lietween 

 the grains l>eing due to the casual meeting of the several groups in the process of 

 growth. 



The references given above will show that there is nothing novel in this view of 

 the structure of metals. Several of the authors' observations may, however, !< 



