FUSION AND SOLIDIFICATION 91 



and assuming an optically perfect surface under 

 the Influence of surface tension. In this way a 

 film Is formed over the surface of a metal, which 

 film is in a state essentially different from that of 

 the bulk of the substance below. Inside the metal 

 the crystalline forces have full play ; at its surface, 

 the controlling influences consist in part of surface 

 tension, which, under the pressure of a polishing 

 tool, is able to overcome the tendency to assume 

 a crystalline structure. In Figs. 19 to 24 are 

 shown six of Sir George Beilby's photographs. 

 Fig. 19 shows the surface of crystalline antimony 

 after rubbing with fine emery paper. The magnifi- 

 cation is such that the photograph Is 775 times 

 life-size. Fig. 20, which represents the same 

 surface after polishing with rouged leather, shows 

 the gradual dragging of a film of metal over the 

 pits and furrows of the first surface. The larger 

 pits get filled with filings of metal, and the film 

 seems to bridge them over, forming a continuous 

 sheet over the loosely-packed fragments below. 

 When an acid or other liquid capable of dissolving 

 the metal is placed on the surface, the film Is dis- 

 solved, and the pits and furrov/s reappear. This 

 comes out In Fig. 21, in which the antimony 

 previously polished has been etched with a solution 

 of potassium cyanide. Fig. 22 shows a polished 

 surface of speculum metal, an alloy used for the 

 reflectors of telescopes. Here the underlying 

 crystalHne structure Is faintly visible. The surface 

 film has, In Fig. 23, been removed with potassium 

 cyanide, and the structure is now plain, the 

 primary crystals, separated by channels of eutectic 

 alloy, being clearly brought out. Finally, in 

 Fig. 24, the same surface has been repolished, 



