SANGER. — VOLATILE COMPOUNDS OF ARSENIC. 131 



flask had been several times replaced. The silver solution, treated as 

 before, gave no arsenic. 



Experiment 3. — The arrangement of flask and tubes, contents of 

 flask, etc., were as in Exp. 2, except that a tube containing cotton 

 wool was used instead of calcic chloride. The action went on for 16 

 days, and at the end of that time there was a slight grayish black 

 precipitate in the silver solution. The latter, however, yielded no 

 arsenic. 



Fermentation on Paper. — Experiment 4. — A large bell jar, having 

 a tubulus at the top and standing on a well greased ground-glass plate, 

 was fitted with a system of tubes and absorbents similar to those of 

 Exps. 1-3. In the jar were placed several pieces of a wall paper con- 

 taining 110 mgr. As.,0 3 per sq. m., giving a total amount of 67 mgr. 

 over a surface of about 0.6 sq. m. The back of the paper was 

 smeared with a paste of flour and syrup, to which part of a yeast cake 

 had been added. Air was drawn through the system for fifteen days. 

 There was apparently considerable reduction of the silver solution 

 and a quantity of black precipitate. This was filtered off, and the 

 filtrate treated as previously described. After a 45 minutes' test of 

 the apparatus, the solution gave, after an hour's run, a scarcely 

 perceptible deposit, which did not look like arsenic, and could not be 

 proved to be arsenic. 



Experiment 5. — The apparatus was the same as in Exp. 4. 

 Another wall paper was taken, having a surface of about 0.89 sq. in. 

 and containing in itself about 45 mgr. As 2 3 . To the flour paste 

 smeared on the back were added about 7 grams arsenious oxide. Air 

 was passed through for 19 days, and at the end of that time the reduc- 

 tion of the silver solution was more marked than in Exp. 4. The fil- 

 tered solution was treated as before, and gave, after 30 minutes, a very 

 slight deposit, not resembling arsenic, and giving no confirmatory test. 



Experiment 6. — This was a continuation of Exp. 5 with the same 

 apparatus and prepared paper, but instead of the silver solution a 

 small quantity of strong sulphuric acid was used as perhaps a better 

 means of absorbing the volatile arsenical compound. Air was drawn 

 through for 15 days. The acid was darkened and sulphur dioxide 

 had been formed. The acid was evaporated with a little nitric acid, 

 diluted, and added to the reduction flask, but no mirror of arsenic 

 appeared in 30 minutes. 



Experiment 7. — This was a continuation of Exp. 5 in exactly the 

 same manner, the sulphuric acid of Exp. 6 being replaced by a silver 



