OP ARTS AND SCIENCES. 37 



treated with sulphuric acid alone. Duplicate readings gave 9.94 mf^r. 

 recovered. 



Finally, 100 sq. cm. of a wall paper containing no arsenic was, 

 after addition of varying amounts of AsgOa, charred with a nitrosul- 

 phuric acid containing one part nitric to thirty of sulphuric acid. 

 The following table shows the results of four analyses. The readings 

 on 2, 3, and 4 were in duplicate. 



1 . . 



2 . . 



3 . . 



4 . . 



/. Effect of Free Nitric Acid in the Reduction Flask. — Rieckher* 

 does not consider the presence of free nitric acid to be detrimental, while 

 Fresenius t takes the opposite ground. Blondlot :j: thinks that free 

 nitric acid gives rise to the formation of a solid hydride of arsenic on 

 the zinc, thus causing the retention of part of the arsenic. Without 

 discussing the correctness of this statement, which seems to have been 

 quite universally accepted, we have only to consider the effect of a 

 trace of the acid on the deposition of the mirror, as the method of 

 charring would leave, at most, but a very small amount of free 

 nitric acid in the extract. 0.02 mgr. and 0.05 mgr. AsoO^ were added 

 to the reduction flasks in which were about 20 c. c. dilute sulphuric 

 acid, and immediately afterward a drop of strong nitric acid was 

 added to each. The dilution was considerable, but not so great as 

 might occur in practice. The mirrors were clearly defined, and cor- 

 responded sharply to 0.02 mgr. and 0.05 mgr. respectively. 



3. Extraction of the " Char." — It is necessary that the char should 

 be thoroughly pulverized and extracted with hot water. Con.siderable 

 loss is likely to occur if these precautions are disregarded, as the fol- 

 lowing results show: — 



To 100 sq. cm. of a wall paper free from arsenic, 1 mgr. AS2O3 was 

 added, and the char was extracted with 30 c. c. cold water. 0.77 mgr. 

 were recovered, = 77 per cent. 



To the same amount of paper 5 mgr. were added, and the char ex- 

 tracted with 30 c. c. cold water. 3.82 mgr. were recovered, = 7G.4 

 per cent. 



* Neues Jalirb. f. Pharm., XX. 3. f Comptes Rendus, LVII. 596. 



t Free. Zeitschr., II. 3b9. 



