46 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



spent in investigating the opalescent phenomena attending the pre- 

 cipitation of traces of argentic chloride. The improved and precise 

 nephelometer* served excellently in this tiresome research. We found 

 that its readings could always be depended upon, and that if any 

 irregularity occurred the fault was with the precipitate within the liquid 

 and not in the means of observing this precipitate. 



The first step in these tests was to prepare a solution of pure 

 argentic chloride, free from excess either of silver or of chlorine. For 

 this purpose pure floccy silver chloride was very thoroughly washed 

 in a flask by decantation. From time to time two nephelometer tubes 

 were filled with the clear decanted wash water ; to one was added a 

 milliliter of argentic nitrate solution, which contained a milligram of 

 silver, and to the other a milliliter of an equivalent solution of sodic 

 chloride. After stirring, these tubes were examined in the nephelome- 

 ter within ten or fifteen minutes, and the intensities of the two opal- 

 escences were compared. No regularity could be detected in theresults, 

 successive trials varying over 50 per cent from one another. For 

 example, in one trial the tube containing an excess of silver was twice 

 as intense in opalescence as the other, while in the following trial, with 

 equal proportions of the same solution, it was only two-thirds as 

 intense as the other. The average of a dozen comparisons confirmed 

 approximately Stas's and Mulder's statement that argentic chloride 

 gives equal opalescence with the two precipitates, but the wide varia- 

 tions between the extremes emphasizes Stas's conclusions as to the 

 uncertainty of the results. 



Subsequent experiments showed that the presence of an electro- 

 lyte greatly accelerated the formation of the precipitate, indicating a 

 colloidal transition stage. The preceding experiments had been 

 almost free from electrolyte, and in view of this inference, it is easy 

 to see why the formation of the precipitate had been so irregular, for 

 slight differences in the extent and speed of mixing might cause impor- 

 tant differences in the coagulation of the colloid. Clearly, in making 

 nephelometric experiments, an excess of electrolyte must be present. 

 Other possible causes of error, such as the effect of light, and the pres- 

 ence of absorbed argentic nitrate or common salt in the precipitated 

 argentic chloride from which the solution was made, were carefully 

 guarded against in the further experiments. In order to avoid pos- 

 sible hydrolysis, f nitric acid was always added. 



Richards and Wells, Am. Chem. Jour, 31, 235 (1904). 



f Mulder, Die Silber-Probirmethode (1859, p. 72). Some of Mulder's observa- 

 tions dn this direction may ihave been due ito occluded material, but the danger of 

 hydrolysis is not wholly to be despised. 



