coloured Films formed by Iodine, fyc. upon Metals. 435 



Bromine "with Bismuth and Silver. 

 Silver . . . pale gold. 

 Bismuth . . . not apparently changed. 

 Silver . . . deep gold, blue. 

 Bismuth . . . yellow, blue. 

 Silver ... blue, yellow. 

 Bismuth . . . dull colourless film. 



Chlorine "with Bismuth and Silver. 



Bismuth is slowly attacked by chlorine gas, much in the 

 same way as with iodine and bromine in vapour. 



Bromine with Lead. 

 At the common temperature neither bromine nor chlorine 

 forms coloured films upon this metal, which it is very difficult 

 besides to bring to any high state of polish on account of its 

 softness. But when lead is heated, as over the flame of a 

 spirit-lamp, the vapours of bromine then form very fine co- 

 loured films, which are in succession gold, deep blue, &c. 



Iodine with Iron. 



These two may be made to form coloured films when com- 

 bined rapidly together, but generally a dull coating without 

 any spectral colour is obtained, on account of the deliques- 

 cence of that salt. 



Until we know the index of refraction of the different 

 films enumerated, it would be impossible to give a correct 

 table of the combining powers in the experiments that have 

 been detailed ; nor is the table of the relative thickness of 

 transparent plates as it has been transmitted to us by Newton, 

 sufficient in the present instance, if any great degree of pre- 

 cision be required. Besides these objections, it is necessary 

 before leaving this subject to pass in review several others 

 inseparable from the mode of performing the experiments 

 themselves. The principal circumstances complicating these 

 experiments and liable to vary in different observations, are, — 



First, the hardness of the metal acted upon ; 2ndly, the 

 obstacle opposed to the continuation of chemical action by the 

 inert film formed upon the metal ; 3rdly, the force of the va- 

 pours that attack the metal. The influence of the texture of 

 the metallic surface on chemical action is most evident when 

 bismuth is the metal employed. Here the chemical action 

 may be seen to commence on small isolated portions of the 

 surface, which have already assumed a deep gold colour, be- 

 fore other parts are in the least changed, from the natural 

 appearance of the metal. To determine how far this might 

 influence the formation of the iodide of silver, a silver coin 



2G2 



