222 GOLD IN SCIENCE AND IN INDUSTBY. 



MOLECULES IN THE SOLID STATE. 



My examination of gold films and surfaces has revealed the fact 

 that during polishing the disturbed surface film behaves exactly 

 like a liquid under the influence of surface tension. At temperatures 

 far below the melting point molecular movement takes place under 

 mechanical disturbance, and the molecules tend to heap up in minute 

 mounds or flattened droplets. These minute mounds are often so 

 shallow that they can only be detected when the surface is illumi- 

 nated by an intense, obliquely incident beam of light. I have esti- 

 mated that these minute mounds or spicules can be seen in this way 

 in films which are not more than 5 to 10 micromillimeters in thick- 

 ness. A film of this attenuation may contain so few as ten to twenty 

 molecules in its thickness. 



AAHien moderately thin films of gold are supported on glass and 

 heated at a temperature of 400 to 500° they become translucent, and 

 the forms assumed under the influence of surface tension can be 

 readily seen Ijv transmitted light. It was in this way that the beau- 

 tiful l)ut puzzling spicular appearance by ol)li(iuely reflected light 

 was first explained as due to the granulation of the surface under 

 the influence of surface tension. Photo-micrograjilis of these films 

 are exhibited. 



Turning now to the mechanical properties of metals, we find that 

 gold has })roved itself of great value in the investigation of some of 

 these. It has long been recognized as the most malleable and ductile 

 of the metals, while its chemical indifference tends to preserve it in 

 a state of metallic purity throughout any prolonged series of opera- 

 tions. 



The artificers in gold must very early have learned that its mallea- 

 bility and ductility are not qualities which indefinitely survive the 

 operations of hammering and wire drawing. A piece of soft gold 

 beaten into a thin plate does not remain equally soft throughout the 

 process, but spreads with increasing difficulty under the hammer. 

 If carelessly beaten, it may even develop cracks round its edges. 

 We may assume that the artificers in gold very soon discovered that 

 by heating, the hardened metal might be restored to its former condi- 

 tion of softness. 



In connection with the study of the micrometallurgv of iron and 

 steel during recent years it has been recognized that heat annealing 

 is as a rule associated with the growth and development of crystal- 

 line grains, and Professor Ewing and Mr. Rosenhain have shown 

 that overstrain is often, if not invariably, associated with the de- 

 formation of these crystalline grains by slips occurring along one 

 or more cleavage planes. This hypothesis, though well supported 

 uj) to a point by microscopic observations on a variety of metals, 

 offers no explanation of the natural arrest of malleability or due- 



