Prof. R. Bunsen on Flame Reactions, 95 



pounds, and the reduction never takes place easily, owing to 

 their slight volatility ; so that it will be better to class them 

 under the groups B 2, B 1, b, as they can be best distinguished by 

 the reactions which are characteristic of these groups. 



8. Behaviour of the Lead Compounds, 



(a) They colour the flame pale blue. 



(b) Reduction film black, dead or brilliant. 



(c) Oxide-film bright yellow-ochre-coloured ; stannous chloride 

 gives no reaction even on addition of caustic soda ; silver-nitrate 

 does not produce any reaction, either alone or on addition of 

 ammonia. 



(d) Iodide-film orange- to lemon-yellow, insoluble on breath- 

 ing or on moistening; disappears on blowing with ammoniacal 

 air, and again appears on warming. 



(e) Sulphide-film brownish red to black; by blowing or moist- 

 ening with ammonium sulphide it remains unaltered. 



(/) On charcoal splinter with soda gives a grey, very soft duc- 

 tile metallic bead, which is slowly but completely soluble in nitric 

 acid, yielding a white easily crystallizable salt, soluble in water, 

 and precipitated as a white powder on addition of sulphuric acid 

 from a capillary tube. 



9. Behaviour of the Cadmium Compounds. 



(a) Metallic film black; the thin parts brown. 



(b) Oxide-film brownish black, shading off through brown to 

 a white invisible film of suboxide, which is not altered by stan- 

 nous chloride either alone or with soda ; silver-nitrate produces 

 a blackish-blue coloration of reduced metal, which is very cha- 

 racteristic and does not disappear on addition of ammonia. 



(c) Iodide-film white, no change produced by ammonia. 



(d) Sulphide-film lemon-yellow, insoluble in liquid ammonium. 



(e) Reduction on charcoal splinter with soda. The metal, owing 

 to its volatility, can only with difficulty be obtained as a silver- 

 white ductile bead. 



10. Behaviour of the Zinc Compounds. 



(a) Reduction film black, in the thin parts brown. 



(b) Oxide film white, and therefore invisible. To test it, a 

 square centimetre of filter-paper moistened with nitric acid is 

 rubbed over the surface and then rolled up on two rings of fine 

 platinum wire, 3 millims. in diameter, and burnt. If the paper 

 is burnt in the upper oxidizing flame at as low a temperature as 

 possible, the ash forms a small solid mass about a square milli- 

 metre in area, which can be ignited without fusion, and becomes 

 yellow on gently heating, appearing, however, white on cooling. 



