306 PROCEEDINGS OF THE AMERICAN ACADEMY. 



cliloride. As we have been careful to eliminate the sulphur before 

 testing, we have not found this necessary, except in certain cases, when, 

 with a sufficiently long deposition tube, it is very simple to insert a 

 strip of paper saturated with normal plumbous acetate and dried. 

 Phosphites and hypophosphites will also have been oxidized before 

 introduction of the solution, and there is little danger in ordinary work 

 from small amounts of phosphine which might result from the acci- 

 dental presence of reducible compounds of phosphorus. Antimony 

 should of course be absent, but very small amounts of stibine do not 

 interfere with the recognition, though they may prevent the estimation 

 of arsenic. Free nitric acid must be avoided. Arseniates require 

 especial treatment, as will be discussed below. 



Special Precautions. 



In order to be certain of uniformity in length and color of the bands 

 from the same amount of solution, the following points must be 

 observed: 



1. The reduction bottles must be of equal capacity and the deposi- 

 tion tubes of equal bore. 



2. The amount of zinc must be the same always, and the granulation 

 must be uniform. 



3. The volume and concentration of the acid must be definite. 



4. The absorbent cotton must be perfectly clean and reasonably dry, 

 and is therefore best stored in a desiccator before use. The amount 

 used should be approximately the same in all cases, packed in the bulb 

 tube to about the same density. 



5. The sensitized paper must be acted upon by a gas in which the 

 moisture is as nearly constant as possible. For this reason the paper 

 cannot be allowed to become moist, nor can the gas be dried. In the 

 first case the band is short and imperfectly shaded ; in the second, it is 

 scattered along the whole length of the strip, or eyen partially escapes 

 the paper. This we have shown by attaching a hard glass tube with 

 capillary, in which, on heating, a mirror of arsenic was obtained. Con- 

 versely, under carefully regulated conditions, no evidence of escaping 

 arsenic was found, either by the use of a hot tube or by the iutroduc"^ 

 tion of a second strip of sensitized paper, 



6. After ten or twelve runs with the same bottle, the atmosphere of 

 the deposition tube becomes too moist, and the bands are consequently 

 too short. ^ It is then necessary to replace the cotton. In order to 

 get a sufficient degree of saturation in the next run, the evolution of 

 hydrogen must go on for a longer time than usual before adding the 



