Prof. R. Bunsen on Flame Reactions. 101 



warming in dilute nitric acid, and yield silver-chloride with hy- 

 drochloric acid, which can then readily be recognized by its be- 

 haviour with nitric acid and ammonia. Less than one-tenth of a 

 milligramme of silver can thus be easily detected with certainty. 



22. Copper Compounds. 



(a) On the charcoal splinter with soda the copper compounds 

 yield a ductile lustrous metallic bead, easily recognizable by its 

 red copper-colour. By rubbing in the mortar, flat metallic par- 

 ticles are obtained, which can be readily washed, and are easily 

 soluble in nitric acid. The blue solution, absorbed on filter- 

 paper, yields a brown stain on addition of potassic ferrocyanide. 

 Instead of acting upon the metal in a curved glass, it may be 

 dissolved by moistening paper upon which it is placed with 

 nitric acid. 



(b) With borax on platinum wire. — Blue bead, not altered to 

 cuprous oxide when heated in the lower reducing flame alone, 

 but on addition of very little tin-salt, forms a reddish-brown 

 bead. If this bead be frequently oxidized and reduced in the 

 flame, a ruby-red transparent bead is obtained ; this occurs most 

 readily when the bead is allowed to oxidize very slowly. 



23. Tin Compounds. 



(a) On the charcoal splinter the tin compounds are easily re- 

 duced to white lustrous ductile metallic beads. The flattened 

 particles transferred to the curved glass slowly dissolve in hydro- 

 chloric acid ; and the solution, when absorbed by paper, gives a 

 red precipitate with selenious, and a black precipitate with tel- 

 lurous acid dissolved in hydrochloric. If to the solution a trace 

 of bismuth-nitrate be added, an excess of soda gives a black pre- 

 cipitate of bismuthous oxide. The metal acted on by nitric acid 

 yields a white powder of insoluble stannic acid. 



(b) A borax bead, containing enough copper-oxide to render 

 it faintly blue, serves as a delicate test to ascertain with certainty 

 the presence of a trace of a tin compound, as the bead placed 

 in the lower reducing flame, as explained above, turns reddish 

 brown, or forms a clear ruby-red glass. 



24. Molybdenum Compounds, 



(a) On a charcoal splinter with soda molybdenum is reduced 

 to a grey powder, but with such difficulty that its detection in 

 this way is not to be recommended. In the same way some mo- 

 lybdenum compounds give in the upper reducing flame a film on 

 porcelain which it is very difficult to obtain. Molybdenum is 

 best detected as follows. 



(b) The sample, finely pulverized with the knife on the porce- 



