368 Scientific Proceedings, Royal Dublin Society. 



however, to condense the vapour and examine the metal which it 

 deposits, and recognize certain metals by the colour and other 

 appearances of the very attenuated films of metal and of metallic 

 oxides which the spark deposits. 



The elements examined so far have been — Gold, Silver, Plati- 

 num, Palladium, Iridium, Copper, Lead, Tin, Arsenic, Antimony, Bis- 

 muth, Thallium, Magnesium, Zinc, Cadmium, Iron, Cobalt, Nickel. 



Gold. — Faraday's deposits of thin films of gold are of two 

 colours, ruby-red and purple ; gold leaf is green. 



Now as the vapour taken from a point of gold by an electric 

 spark must be extremely attenuated, and very minute in quantity, 

 the only chance of recognizing it is to cause it to deposit on some 

 transparent surface. Sparks passed over thin films of mica deposit 

 on the surface faint bluish streaks of extreme beauty, like the 

 bloom on a grape. Occasionally the palest possible rose-colour 

 is spread over the .surface. The appearances, which at first sight 

 are not easily discernible, are plainly visible even when caused by 

 a single spark, when the eye is familiar with them. To increase 

 their visibility, they were made to strike the polished surface of a 

 piece of platinum foil, which was supported horizontally between 

 the points, the path of the spark being vertical. After five 

 minutes the appearance of the metal was altered so that the polish 

 was destroyed on both surfaces, but there was no evidence by its 

 colour of a deposit of gold, but minute globules of metal were 

 visible. It seemed probable that the roughness on the under 

 surface was caused by the vapour of gold being condensed on the 

 under surface, and that the platinum melted and volatilized when 

 the sparks sprang off at the other side. The metal was not per- 

 forated, as was the case with tin-foil. By using a very thin film 

 of mica the sparks at first traversed the surface horizontally in 

 every direction, sometimes springing over the edges ; but they 

 soon perforated the film, and a succession of sparka during four 

 minutes deposited on the under surface a very beautiful film. The 

 hole is rounded, and the mica is fused : around this a circular de- 

 posit, from half an inch to an ] inch in diameter, is seen of an ex- 

 quisite blue colour, verging near the outer edge upon green (pi. vi.). 

 The part towards the centre is more or less opaque, bat a fine 

 ruby or rose-red is visible. Just around the perforation reflected 

 light shows a fine deposit of gold of a beautiful reddish colour 



