94 Prof. R. Bunsen on Flame Reactions. 



6. Behaviour of the Mercury Compounds. 



(a) Metallic film is mouse-grey, non-coherent, and spreads 

 over the whole basin. To obtain small traces of mercury in the 

 reduced state, the sample is mixed with soda and saltpetre and 

 filled into a thin test-tube 5 to 6 millims. wide and 10 to 20 

 millims. long. This is held by a platinum wire in the flame, 

 whilst the bottom of the basin filled with cold water is placed 

 close above the open end of the tube. If the quantity of mer- 

 cury is considerable, the metal collects in the form of small glo- 

 bules which can be seen with a lens, and which can be collected 

 into larger drops by wiping the basin with a piece of moistened 

 filter-paper. 



(b) Oxide-film cannot be obtained. 



(c) Iodide-film is obtained by breathing on the metallic film 

 and then placing it over the vessel (fig. 8) containing moist bro- 

 mine. It first becomes black and then disappears, but not until 

 after some time, as mercuric bromide is formed. If the basin be 

 now placed above the vessel of fuming hydriodic acid, a very cha- 

 racteristic carmine-coloured film of mercuric iodide is produced : 

 this is often accompanied by mercurous iodide ; but neither of 

 these disappear on breathing, nor when blown with ammoniacal air. 



(d) Sulphide-film black, not altered by breathing or by blow- 

 ing with ammonium- sulphide. 



(e) With soda on the charcoal splinter gives no reaction. 



7. Behaviour of the Thallium Compounds, 



As the presence of a few millionths of a milligramme of thal- 

 lium can be recognized by the spectroscope, it does not often 

 happen that this metal needs to be sought for in any other way. 

 I add the following reactions for the sake of completeness. 



(a) Flame -coloration bright grass-green. 



(b) Metallic film black ; thin parts coffee-coloured. 



(c) Oxide-film colourless ; with stannous chloride and soda, 

 as well as with silver-nitrate with and without ammonia,, no 

 alteration. 



(d) Iodide-film lemon-yellow, does not disappearon breathing, or 

 on blowing with ammoniacal air, or on addition of liquid ammonia. 



(e) Sulphide-film formed from the oxide, black, and the thin 

 parts bluish grey ; insoluble in liquid ammonium-sulphide. 



(/) On charcoal splinter with soda reduced to a white ductile 

 bead, which quickly oxidizes in the air, and is acted upon by 

 hydrochloric acid with difficulty. ^ 



In addition to the foregoing, there are two metals, viz. tin and 

 molybdenum, which may be classed under the group A 2. These 

 two metals, however, cannot be reduced from all their com- 



