THE STRUCTURE OF METALS 103 



cracked at the edges when an attempt was made to hammer 

 it out into a disc long before a specimen of pure copper 

 would have shown signs of failure. The effect of impurities 

 on the mechanical properties of copper is profoundly modified 

 by the simultaneous presence of oxygen, a fact well known 

 to metallurgists. 



The detection of impurities is thus a very important part 

 of the work of the metallographist and the chemical and micro- 

 scopical methods supplement one another in a most valuable 

 way in indicating the properties that may be expected from 

 a given metal or alloy. It must not be forgotten, however, 

 that microscopical examination also gives information which 

 it is not in the power of any chemical analysis to yield — namely, 

 in respect to the heat-treatment that a metal has undergone, 

 on which its physical and mechanical properties so largely 

 depend. Widely different results may be obtained from two 

 specimens of identical chemical composition but the micro- 

 scopical method seldom fails to throw some light on the 

 difference. Naturally, the relation between structure and pro- 

 perties has not been by any means equally determined in the 

 case of all alloys and there are still many obscure and uncer- 

 tain points in the method. But both the technical details of 

 manipulation and the establishment of definite relations are 

 advancing rapidly and the microscope is becoming more and 

 more indispensable in all departments of metallurgy. Famili- 

 arity with the method is necessary in order to utilise its 

 indications and it is only possible in a short notice to touch 

 upon a few prominent points. The highly important subject 

 of the effect on metals of mechanical deformation is reserved 

 for a second article. 



