SANGER AND BLACK. — DETERMINATION OF ARSENIC IN URINE. 339 



this residue was oxidized after a very long time, so that it appeared 

 nearly colorless. The diluted residue was then added to the reduction 

 bottle. A series of analyses was made by this method in which the 

 amounts of arsenic were not known to the analyst. 



SERIES E. 

 Nitric Acid Method, Modified. 



From comparison of Series D and E, it will be seen that the distilla- 

 tion method is more accurate than the evaporation method, even if 

 the latter is carefully conducted so that the loss from carbonization is 

 avoided as far as possible. But the time needed for a proper treatment 

 with nitric acid by the latter method is very great, and the manipula- 

 tion uncleanly. The entire preparation of the solution for testing, in 

 the distillation method, does not consume more than three fifths of the 

 time required in the other, and the procedure is much cleaner. 



Use of the Method. 



We have not studied the question of how small an amount of 

 arsenic can be recovered from the urine by this method, but have been 

 content to show that very small amounts, even as little as 0.01 mg. 

 per liter, can be detected and estimated with reasonable accuracy. 

 (Series A and Analysis No. 39.) For the examination of abnormal 

 urine, — in studying the elimination of arsenic through the kidneys, 

 for instance, — it would be seldom necessary to consider a quantity 



