1902.] A Note on the Eecrystallisation of Platinum. 253 



characteristic of the etched surface of a crystalline metal. The 

 mkrographic appearance under oblique light is shown, magnified 30 

 diameters, in the photograph (fig. 1). This was taken from the surface 



Fig. 1. Surface of a Platinum Crucible after prolonged heating. Oblique 

 light. Magnification, 30 diameters. 



-of a platinum crucible which has been in continuous use and is fre- 

 quently exposed for hours together to an ordinary blowpipe flame. 

 A closer examination revealed a multitude of geometrical pits (Aetzfi- 

 guren), clearly demonstrating that the appearance is due to genuine 

 metallic crystals which have been etched on the surface by some 

 chemical agent. Two other experimental facts confirm this idea. The 

 first is that if a piece of platinum showing the " changed " surface be 

 exposed to the action of aqua-regia, the appearance is intensified and 

 brightened. If, as has been suggested, the appearance were due to a 

 superficial layer of a platinum-carbon compound from which the carbon 

 had been driven off, leaving mere pseudomorphs of platinum, the 

 etching action of the aqua-regia would have dimmed the appearance 

 instead of brightening it. Another objection to the carbonisation 

 theory is to be found in the fact that I have produced the effect on a 

 new platinum crucible by prolonged heating in an oxygen injector 

 furnace, where an oxidising atmosphere was being maintained. 



The " changed " platinum is, moreover, very weak and brittle when 

 hot, and on one occasion a crucible was torn whilst still red hot, but 

 after being removed from the flame. The fracture was as crystalline 



