232 Mr. G. T. Beilby. The Effects of [June 4, 



not merely in one but in all directions. This loss pointed to increased 

 absorption, and the first idea was that the escaping vapour had blown 

 the molten alloy into multitudes of films so thin as to be devoid of 

 reflecting power. But this view was not borne out by the behaviour 

 under mercury treatment of films supported on glass. In their case 

 it was found that the metal left, after the mercury had been driven off, 

 had arranged itself in transparent granular forms of greatly diminished 

 reflecting power (fig. 28). The loss of light then is mainly due to 

 absorption by the more transparent metal, not to scattering by 

 repeated reflection from films. 



A knowledge of this fact is of value in interpreting the appearances 

 presented by surfaces of gold etched by aqua regia or by chlorine. In 

 this case also there is an immense loss of light, which cannot be 

 caused by scattering reflection, and can only be due to increased 

 absorption. 



Fig. 25 is a spot on a gold plate which has been amalgamated with 

 mercury and then heated. On the light part of the surface the 

 mercury has acted very slightly, and the lustre of the original surface 

 is not much diminished, though there has been sufficient action to 

 bring about aggregation in rounded forms. On the dark part the 

 action has gone deeper, and in consequence the amount of light which 

 has escaped absorption has been insufficient to disclose the details of 

 structure. 



Figs. 26 and 27 are photographs of the light and dark parts at a 

 higher magnification by the 3 mm. objective. In Fig. 27 the spots 

 of light on the dark back ground are reflections from the tops of 

 transparent or translucent granules of rounded form. 



The foregoing observations on the effects of heat on thin films and 

 on surface films show conclusively that considerable molecular re- 

 arrangement is brought about at a temperature very much below the 

 melting point of the metal. The mere fact of there being a certain 

 amount of freedom imparted to the molecules by heat is not surprising, 

 in view of the well-established facts of segregation and crystal growth 

 in masses of metal at temperatures much below their melting point. 

 But that this freedom takes place under conditions which lead the 

 solid molecules to behave like liquid molecules is both new and 

 suggestive. 



The appearance of viscous flow and the forms assumed by the 

 surface film suggest neither segregation nor crystal growth, but they 

 do strongly suggest the behaviour of a viscous fluid under the control 

 of surface tension. It appears probable, therefore, that at the surface 

 of a solid the crystallic force is controlled and kept in check, some- 

 times actually overpowered, by surface tension, although in the body 

 of the metal the equally balanced molecular attractions do not 

 effectually oppose the crystallic. 



