Prof. R. Bunsen on Flame Reactions. 89 



or brilliantly-black films of varying thickness. Even lead, tin, 

 cadmium, and zinc yield in this way films of reduced metal which 

 by mere inspection cannot be distinguished from the soot sepa- 

 rated out on the porcelain by a smoky flame. By means of a 

 glass rod, these films can be touched with a drop of dilute nitric 

 acid containing about 20 per cent, of real acid; and the various 

 degrees of solubility of the films serves as a distinguishing cha- 

 racteristic. 



(b) Oxide-films are obtained by holding the porcelain basin 

 filled with water in the upper oxidizing flame, the rest of the 

 operation being the same as in the production of the metallic 

 films. If only a very small quantity of the sample can be em- 

 ployed, care must be taken to lessen the size of the flame, in 

 order that the volatile products may not be spread over too large a 

 surface of porcelain. 



The film of oxide is examined as follows : — 

 (a) The colour of the thick and thin film is carefully observed. 

 (/3) The reducing action or otherwise of a drop of stannous 

 chloride is noted. 



(7) If no reduction occurs, caustic soda is added to the stan- 

 nous chloride until the precipitated hydrate redissolves, and then 

 it is to be observed whether a reduction occurs. 



(8) A drop of perfectly neutral silver-nitrate is rubbed over the 

 film with a glass rod, and a current of ammoniacal air is blown 

 over the surface from a small wash-bottle containing ammonia 

 solution, and having the mouth-tube dipping under the liquid 

 and the exit-tube cut off close below the cork. If a precipitate 

 is formed, the colour is observed, and the solubility or alteration, 

 if any, noticed which occurs when the current of alkaline air is 

 continued, or when a drop of ammonia-liquor is added. 



(c) Iodide-films are simply obtained from the oxide-films by 

 breathing on the latter upon the cold basin, which is then placed 

 upon the wide-mouthed well-stoppered glass (fig. 8) containing 

 fuming hydriodic acid and phosphorous acid derived from the 

 gradual deliquescence of phosphoric triiodide. When the mix- 

 ture no longer fumes, owing to absorption of moisture, it is easy 

 to render it again fuming by adding a little anhydrous phosphoric 

 acid. Other films, often containing both iodides of a metal, and 

 therefore frequently less regular in colour and appearance, may 

 be easily obtained by smoking the oxide-film with a flame of alco- 

 hol containing iodine in solution placed upon a bundle of asbestos- 

 threads and held under the basin. If any iodine be condensed 

 on the basin with the hydriodic acid, it can easily be removed by 

 gentle warming and blowing. 



The examination of the film is conducted as follows : — 



(a) The solubility of the film is examined simply by breathing 



