- OF THE AMEBICAM A.CADEMY. 



have been careful to eliminate the sulphur before 



found this necessary, except in oertau when, 



w jth Qtly long deposition tube, it is very -imple to inert a 



• 1 with normal plumbous acetate ami dried 

 Phosphites and bypophosphites will also have been oxidized before 

 int! if the solution, and there is little danger in ordinary work 



tall amounts of phosphine which might result from the acci- 

 dental • ■' of reducible compounds of phosphorus. Antimony 

 dd of course be absent, bul very small amounts of Btibine do not 

 ith 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 Peecautioi 



In order to he certain of uniformity in length ami color of tin 1 bands 

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

 observed: 



1. Tin' reduction buttles must be of equal capacity and the deposi- 

 tion tubes of equal bore. 



•J. Tin' amount of zinc must be the same always, and the granulation 

 must be uniform. 



:;. The volume and concentration of the acid must be definite. 

 I. TL' absorbent cotton must he perfectly clean ami reasonably dry, 

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

 I Bhould he approximately the same in all cases, packed in the bulb 

 tube to about the same density. 



.".. 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 

 not be allowed to become moist, oor can the gas be dried. In the 

 fire! the band is short and imperfectly shaded \ in the second, it is 



ed along the whole length of the strip, or even partially escapes 

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

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

 versely, under carefully regulated conditions, no evidence of escaping 

 • found, either by the use of a hot tube or by the introduc- 

 tion of ud 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 i on • • [uently 



It i< then necessary to replace the cotton. In older to 



ifficienl degree of saturation in the next run, the evolution of 



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



