106 Prof. R. Bunsen on Flame Reactions. 



A drop of neutral silver solution is now brought on to the residue 

 at the moment when it becomes solid ; and on blowing with arn- 

 moniacal air, a characteristic black stain is formed, whilst the 

 reaction of arsenic is also generally noticed. 



In order to detect tin, a few scarcely visible particles of the 

 roasted sulphides are fused on to a borax bead which has been 

 very slightly tinted with cupric oxide. If the bead is now 

 brought into the lower reducing flame, it becomes a ruby-red 

 colour from reduced cuprous oxide. If the oxide be present 

 in too large a quantity, the bead can be obtained transparent 

 by the process described under the reactions of the copper com- 

 pounds. This reaction can only be obtained in the lower redu- 

 cing flame of the non-luminous gas-lamp, as in the ordinary 

 blowpipe-flame the cupric oxide is reduced to cuprous oxide 

 without the presence of tin-salt. 



2. Black tellurium, containing tellurium, selenium, antimony, 

 lead, gold, and sulphur. 



After the sulphur has been detected by the smell on roasting, 

 the metallic film is obtained on a test-tube, which is then placed 

 inside a wider and shorter tube containing a few drops of con- 

 centrated sulphuric acid, so that the metallic film is surrounded 

 by the acid. If the temperature be now gradually raised, the 

 presence of tellurium is at once ascertained by the formation of 

 a bright carmine-colour. If the temperature be still further 

 raised, the tellurium oxidizes, and the olive-green colour of 

 selenium becomes visible; the cooled solution, on dilution with 

 water, then no longer exhibits the black precipitate of tellurium, 

 but is coloured yellowish red from the selenium. If this sub- 

 stance be present only in small traces, it can be best detected by 

 looking down the length of the test-tube upon a sheet of white 

 paper. As the common commercial sulphuric acid not unfre- 

 quently contains traces of selenium, it is well to make a blind 

 experiment to assure one's self of its absence. The antimony is 

 detected exactly as described in the preceding example. To 

 detect the lead and gold, a sample is reduced on the charcoal 

 splinter, the beads of the alloy are washed into a curved glass, 

 and the flattened and dried metallic particles treated with pretty 

 strong nitric nitric acid as long as anything dissolves. The 

 acid is then evaporated off, and the soluble portion of the residue 

 dissolved in a drop or two of water. The solution is then 

 brought on to a second curved glass by means of a capillary 

 pipette, and the characteristic precipitate of lead- sulphate ob- 

 tained by addition of a few milligrammes of sulphuric acid. The 

 gold, left undissolved as a brown powder, is completely washed by 

 frequent addition of water and removal of the same with the 



