308 pkoceedings of the american academy". 



Analytical Data. 



The method, as far as it concerns the determination of arsenic in a 

 solution properly prepared for reduction, was tested by the analyses 

 of solutions containing varying amounts of arsenic, which, with the ex- 

 ception of Nos. 5, 8, 7, and 9, were unknown to the analyst (see Table I). 

 In analysis No. 9 the arsenic was present as arsenic acid. In Nos. 5, 8, 

 7, and 9, the comparison was made with standards which had been kept 

 over three months, and the reading of the bands was confirmed by the 

 standards obtained by development of the initial bands with ammonia. 



We do not claim for the method, under ordinary circumstances, a 

 greater accuracy than from five to ten per cent. 



Analytical Notes. 



Sensitized Paper. We have found that the prepared paper, if kept 

 dry and away from the light, does not lose its sensitiveness to a great 

 extent after several months. On long keeping there is apparently a 

 very slight reduction to mercurous chloride, since an old paper after 

 treatment with hydrochloric acid and washing gives a slight darkening 

 with ammonia or auric chloride (for this test, see below). Although 

 this change does not greatly influence the result, it is better not to use 

 paper which has been kept too long. 



Contrary to Goode and Perkin,^^ we have found no advantage in 

 using mercuric bromide instead of the chloride. Neither the aqueous 

 solution of the former, which is, in addition, too dilute, nor the alco- 

 holic solution, gives a paper of greater sensitiveness than that prepared 

 from the chloride. The alcoholic solution of the chloride, since it 

 evaporates more rapidly, leaves a less even surface of the salt upon 

 the paper than is obtained by the slower evaporation of the aqueous 

 solution. 



Apparatus. In case it is necessary to examine larger quantities of 

 solution for arsenic, a larger reduction bottle will naturally suggest 

 itself in this case, slight variations from the procedure may be found 

 necessary, and the absolute delicacy of the method may be some- 

 what less. 



We have found no sign of arsenical contamination from the rubber 

 stoppers used in the apparatus, and we have therefore not lessened the 

 simplicity of the apparatus by making it entirely of glass. The stop- 

 pers are boiled with dilute alkali and washed before use. 



" Loc. cit. 



