Hartley — On the Constitution of Electric Sparks. 369 



with metallic lustre ; slightly removed from this a greenish tinge, 

 irregularly disposed, is seen, while here and there yellow and 

 brilliant films spread till their tenuity does not admit of light 

 being reflected. The appearance is most beautiful when viewed 

 under the microscope with a power of 100 diameters. Around 

 the perforation and at a slight distance, or even upon its edge, 

 the mica is encrusted with globules of gold. The faint bluish 

 tiuge may be traced over an area equal to at least a square inch, 

 but it is circular in form. There is no evidence to show why the 

 thinnest films are sometimes red, and at other times blue. The 

 same differences in colour are to be observed even where the gold is 

 thickest. All cracks and fissures in the mica are filled with gold, 

 even in places where none is condensed upon the smooth surface. 



Silver. — Silver forms a film which is of varying shades of 

 yellow, amber, and even rose-coloured or violet, the rose tint 

 appearing in patches where the film is very thin. At the edge of 

 the perforation a deposit of frosted silver is seen by reflected light, 

 and this is either brown or quite opaque. The microscope reveals 

 that the opaque incrustation consists of minute globules of silver. 

 The area covered by the incrustation and film is similar in size 

 to that formed by the gold, though the colour of the gold, being 

 much stronger and more peculiar, can be traced over a larger 

 area (pi. vi.). 



Platinum. — Platinum yields a smaller sublimate of an intensely 

 deep rich-brown colour, showing that it is less volatile. By re- 

 flected light its metallic lustre and characteristic colour appear, 

 and the microscopic appearance is that of an aggregate of minute 

 globules of the metal. 



Palladium. — Palladium yields a film very similar to that of 

 platinum as to colour, but it covers a much larger area, the 

 diameter of the outer circular sublimate being at least twice as 

 great as that of platinum. 



Iridium. — Iridium condenses in a manner very similar to that 

 of platinum, but the colour is strikingly different, being of a 

 beautiful grey. The deposit of metal is smaller, but the difference 

 is not very marked. 



Copper. — Copper forms a black, and at the edges a brown, 

 film, but there is no metallic lustre visible except on using the 

 microscope ; the globules, which are very minute and sparsely 



