THE PRECIPITATION AND WEIGHING OF ARGENTIC CHLORIDE. 19 



stages, just as in the often-cited investigation of sodium. The first stage 

 included the mother liquor and first five wash-waters, containing inten- 

 tionally as much argentic nitrate as the mother liquor, and added to remove 

 the greater part of the potassic nitrate. This first quantity of liquid was 

 entirely free from chlorine in every accepted analysis, as was shown by 

 careful testing in the nephelometer, and so was at once cast aside. 



The second stage of washing was conducted with very dilute nitric 

 acid, and yielded 0.5 or 0.6 liter of a very dilute solution of mixed nitric 

 acid and argentic chloride and nitrate. The traces of chloride which it 

 contained were carefully estimated in the nephelometer after addition 

 of excess of argentic nitrate. 



The third stage of the washing, also conducted with pure water acidified 

 with nitric acid to prevent colloidal irregularities, yielded a liter or more 

 of liquid containing much argentic chloride. The analysis of this liquid 

 constituted a very important part of the work, and gave much trouble. 



In order to be as certain as possible of the weight of chlorine it con- 

 tained, this liquid was tested in five cases according to two very different 

 methods. The first of these consisted in evaporation of the solution 

 under diminished pressure and actual weighing of the precipitate, and 

 the second consisted in a new modification of the nephelometer test. 

 Although the latter evidently gave the more satisfactory results, the 

 former is worthy of brief discussion, because its outcome certainly repre- 

 sents the maximum value. 



In order to carry out these two parallel quantitative tests, the collected 

 wash-water was divided into two parts ; and the larger part, about 90 

 per cent of the whole, was evaporated under low pressure at 50 in a 

 specially made Jena glass 2-liter flask with a glass stopper. In the course 

 of this evaporation the argentic chloride began to separate' out when the 

 volume had been reduced to about 0.2 liter, if 2 mg. of the salt were 

 present a fact agreeing well with the recent observations concerning this 

 solubility. 1 The dried residue was dissolved in a very little ammonia 

 freshly distilled in platinum, and washed into a small precipitating flask, 

 where it was reprecipitated with excess of argentic nitrate and nitric acid. 

 This trace of precipitate was collected and weighed on a fresh Gooch- 

 Munroe crucible in the same way as the larger mass. The results are 

 given below in comparison with those obtained from the same solutions 

 by the nephelometer. 



An important modification was introduced into the nephelometric deter- 

 mination. One of us has repeatedly pointed out that in order to obtain 

 satisfactory results with this instrument the solution to be tested and 

 that used as the standard of comparison must be treated in exactly the 



1 See for example Bottger, Zeitschr. phys. Chem., 56, 93 (1906). 



