90 PHYSICAL SCIENCE 



the most part distinguished, the crystals of each 

 constituent having a characteristic appearance, 

 which usually persists throughout a series of 

 changes. 



The investigations we have described all 

 emphasise one point — the fact that metals possess 

 a structure essentially crystalline. In some cases, 

 such as that of the surfaces of zinc deposited on 

 so-called galvanised iron, this crystalline structure 

 is readily visible, but most of the metallic objects 

 in common use possess polished surfaces on which 

 no trace of crystals can be seen. The possibility 

 of polishing a surface to such a state of perfection 

 that it will act as a mirror and reflect a ray of 

 light without appreciable scattering, is a matter 

 of considerable interest. Any irregularities on 

 such a surface must be small compared with the 

 wave-length of light, and it is difficult to see how 

 any such surface could be obtained by the use of 

 ordinary polishing materials, if the action of these 

 materials be regarded as a mere mechanical 

 grinding away of projections after the manner 

 of a file. 



Many careful observations have been made on 

 the process of polishing. Among them should 

 be noted those published in August 1903 in the 

 Proceedings of the Royal Society, by Sir George 

 Beilby. He investigated the subject microscop- 

 ically, and found reason to believe that the passage 

 over the surface of a scratched metal of a polish- 

 ing substance like wash leather covered with rouge 

 produces a kind of surface flow, the outer layers 

 of the metal flowing like a viscous liquid under 

 the action of the pressure on the polishing tool, 



