1857.] of Gold (and other Metals} to Light. 399 



tinuity of the metallic film up to the very edges of these real 

 apertures. When pressure is applied to this translucent film, 

 the compressed metal becomes either opake or of a very dark 

 purple colour, and resumes its high reflective power. If a 

 higher heat than that necessary for this first change be applied, 

 then the leaf, viewed in the microscope, assumes a mottled 

 appearance, as if a retraction into separate parts had occurred. 

 At a still higher temperature this effect is increased ; but the 

 heat, whether applied in the muffle or by a blowpipe, which is 

 necessary to fuse the metal and make it run together in globules, 

 is very much higher than that which causes the first change of 

 the silver : the latter is, in fact, below such a red heat as is 

 just visible in the dark. Whatever the degree of heat applied, 

 the metal remains as metallic silver during the whole time. 

 When many silver leaves were laid loosely one upon another, 

 rolled up into a loose coil, introduced into a glass tube, and 

 the whole placed in a muffle and heated carefully for three or 

 four hours to so low a degree that the glass tube had not been 

 softened or deformed 5 it was found that the silver-leaf had 

 sunk together a little and shaped itself in some degree upon the 

 glass, touching by points here and there, but not adhering to 

 it. But it was changed, so that the light of a candle could be 

 seen through forty thicknesses : it had not run together, though 

 it adhered where one part touched another. It did not look 

 like metal, unless one thought of it as divided dead metal, and 

 it even appeared too unsubstantial and translucent for that ; 

 but when pressed together, it clung and adhered like clean 

 silver, and resumed all its metallic characters. ^ 



When the silver is much heated, there is no doubt that the 

 leaf runs up into particles more or less separate. But the 

 question still remains as to the first effect of heat, whether it 

 merely causes a retraction of the particles, or really changes 

 the optical and physical nature of the metal from the beaten or 

 pressed state to another from which pressure can return it back 

 again to its more splendid condition. It seems just possible 

 that the leaf may consist of an infinity of parts resulting from 

 replications, foldings and scales, all laid parallel by the beating 

 which has produced them, and that the first action of heat is 

 to cause these to open out from each other ; but that supposi- 

 tion leaves many of the facts either imperfectly explained or 



