52 MODERN SCIENCE OF METALS 



applied to these pieces of extra-terrestrial metal 

 the methods of microscopic examination which he 

 had successfully developed for the study of rocks. 

 The nature of metals that they cannot be cut 

 into sections or slices sufficiently thin to be trans- 

 parent, so that the optical methods of petrography 

 cannot be applied to them, forced Sorby to develop 

 new and beautiful methods for their microscopic 

 examination. He thus evolved the technique of 

 preparing metal surfaces by first polishing them 

 very perfectly and then "etching" them by 

 attacking them with an acid. This mode of 

 attack produces a surface pattern of great clearness 

 and beauty which reveals the internal structure 

 of the metal. This method has formed the founda- 

 tion for one of the most important branches of 

 the New Metallurgy, but how little its practical 

 importance was realised even in Sheffield which 

 claims to be the home of all that is best in Metal- 

 lurgy is shown by the fact that Sorby's work 

 lay neglected and forgotten until it was revived 

 by the independent initiative of two continental 

 workers Osmond in France and Martens in Ger- 

 many. Yet at the present time, every metallurgical 

 works of any importance has its metallographic 

 laboratory where the methods of Sorby are daily 

 practised for the control of industrial operations. 



While the new instrument of investigation which 

 was placed in the hands of metallurgists by Sorby 

 has contributed very largely to the growth of the 



