316 PROCEEDINGS OF THE AMERICAN ACADEMY. 



From these results it will be seen that if we have a color hand from 

 pure material, within or above the range of the 4 mm. arsenic stand- 

 ards, the differentiation of arsenic from antimony, phosphorus, and 

 sulphur is perfectly simple. With smaller amounts, or especially with 

 mere traces, there can be no confusion with antimony, since stibine 

 gives no yellow color on the paper. With sulphur, while the small 

 initial band might be mistaken for arsenic, the treatment with hot 

 water, ammonia, and auric chloride will easily identify it. But with 

 phosphorus there is likely to be a doubt if the 2 mm. band ^9 is very 

 small, since the amount and length of the color do not permit the 

 same comparison as in the larger bands. As we have shown, however, 

 that even as much as 0.1 mg. of phosphorus gives very little color in 

 thirty minutes of reduction, and as this is a quantity which can be easily 

 oxidized in the preparation of the solution for analysis, we should have 

 little to fear from smaller amounts than 0.1 mg. Such amounts might 

 be considered quite accidental. 



Effect of Hydrogen Sulphide, Phosphi7ie, or Stibine on the Arsenic 

 Band. Very ditt'erent is it, however, when the arsenic solution also 

 gives by reduction as much of any one of these gases as would alone 

 yield a band equal to the arsenic band in length. This is shown by the 

 following experiments. 



Hydrogen Sulphide. Amounts of the respective solutions, equal to 

 30 mmg. of arsenious oxide and 50 mmg. of sulphur, were added to- 

 gether to a bottle and reduced for thirty minutes. Instead of the short, 

 well-defined band of the arsenic, a band nearly three quarters of the 

 length of the strip was formed, of a reddish yellow color. Hydrochlo- 

 ric acid turned it slightly redder, but the appearance was not definitely 

 characteristic of arsenic. On another similar band ammonia brought 

 out splotches of black on a red ground. The arsenic had evidently 

 acted as an accelerator in the reduction of the sulphurous acid, and the 

 resulting band was due to a mixture of the arsenic and sulphur com- 

 pounds, spread over a greater surface. 



Phosphine. Solutions containing 30 mmg. arsenious oxide and 

 200 mmg. phosphorus were added to a bottle and reduced for thirty 

 minutes. The band was longer than the corresponding band of ar- 

 senic, but with the characteristic appearance of the latter, — well 

 shaded, except that it was somewhat lighter at the top. Hydrochloric 

 acid converted the color to the well-marked red of arsenic and the 

 length agreed with the hydrochloric acid standard for 30 mmg. Auric 



29 For the use of tlie 2 mm. band, see below. 



