140 SPECIAL METHODS OF ANALYSIS. 



If copper was not found, take only a small part of 

 F. 3 for this trial. 



Care must be taken not to inhale the gas from the 

 flask, if there is any reason to suspect that arsenic 

 is present. 



After the hydrogen has been evolved a few min- 

 utes, wrap a towel around the flask, ignite the jet 

 of gas, and hold a cold porcelain surface in the 

 flame. Black lustrous spots are deposited on the 

 porcelain surface where the flame comes in contact 

 with it. Arsenic. 13 



13. a. Copper was not found, and only a small portion 



of F. 3 was taken for the test in 12. - - - 14 

 t. Copper was found, and a Jarge portion of F. 3 

 is under examination. Allow the action of the 

 zinc to continue 10-15 minutes, or until all the cop- 

 per is precipitated ; a much longer time may be re- 

 quired if the solution contains a notable quantity 

 of nitric acid ; finally filter the liquid from the pre- 

 cipitated metal, and mark the filtrate F. 13. - - 14 



14. Add a little nitric acid to a portion of F. 13, or of 

 F. 3, if there is no F. 13 and the first solution did 

 not already contain free nitric acid in excess, and 

 heat the mixture to boiling. 



a. An unmistakable reaction for iron was obtained 



in 11. 15 



h. Not. To a small portion of this solution add 

 potassic sulphocyanate. A deep red color appears. 

 Iron. ----.-•.. 15 



15. To a large portion of the solution obtained in 14, 

 add amnionic carbonate in excess. 



a. The precipitate which may have been formed 

 at first is entirely re-dissoUed by the excess of the 

 reagent, and the solution remains quite clear, even 

 after a hot digestion of 12 hours. - - - 17 



