THE STRUCTURE OF METALS 91 



Under a high magnification the roughening is seen to be due to 

 the formation of very numerous "etch-figures" or hollows of 

 geometrical outline ; the form of these serves to give informa- 

 tion as to the crystalline system to which the metal belongs. 

 Within any one grain, the arrangement of the etch-figures is 

 strictly parallel but the orientation varies from grain to grain, 

 the result being that when light falls on the etched surface it is 

 reflected at different angles by different grains, so that one may 

 appear light and another dark in the field of the microscope. 

 The boundaries of the grains thus become visible as boundaries 

 of light and shade. Another circumstance contributes to render 

 the structure visible. Etching takes place more rapidly at the 

 boundaries than elsewhere, so that after a short time the grains 

 are separated by grooves which become broader and deeper on 

 longer etching. The cause of this phenomenon is not quite 

 clear. Traces of impurity would tend to accumulate at the 

 bounding surfaces of the polyhedral grains and would be re- 

 moved by etching ; but the effect is produced in the most care- 

 fully purified metals. It is most probable that the acid acts with 

 different degrees of rapidity along different planes in the crystal 

 — the fact that etch-figures are formed, indeed, points to such 

 a conclusion — and the junction between two grains of different 

 orientation may thus give rise to a difference of electrolytic 

 potential which is small but sufficient to produce an increased 

 action at the boundary. The photograph of iron containing 

 only very small quantities of impurities (" American ingot iron," 

 really a mild steel almost free from carbon) shown in fig. 1 is 

 a typical example of the structure obtained on casting a homo- 

 geneous metal. The etching has been so light that the surfaces 

 of the crystal grains have hardly been roughened and the 

 structure has only been rendered visible on account of the etch- 

 ing at the junctions of the grains producing a fine groove which 

 is visible as a dark boundary line. 



If, instead of a single metal, the mass under examination be 

 an alloy, cases may occur in which the structure observed in a 

 slowly cooled ingot does not differ from that just described. 

 Yellow brass, containing 70 per cent, of copper and 30 per cent, 

 of zinc, is an example of such an alloy. The brass contains 

 only a single micrographic constituent, as the copper is capable 

 of retaining the whole of the zinc in a state of uniform ad- 

 mixture. Apparent homogeneity in each crystal grain is reached, 



