28 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



with this instrument the average of many results is within i or 2 per 

 cent of the truth, so that the error with 2 milligrams of dissolved 

 argentic chloride is probably less than 0.04 milligram. This degree 

 of accuracy is as great as can be obtained in the collection of a pre- 

 cipitate for weighing, hence the method is to be ranked among those 

 suitable for exact work. 



With the perfected nephelometer it was possibly to study the pro- 

 gressing precipitation of very dilute argentic chloride in a way pre- 

 viously impossible, and several unexpected irregularities were found 

 during its course. Since these are important in the quantitative sense, 

 they must all be briefly discussed. 



The most striking of these is the variation in! the time of complete 

 precipitation caused by the presence of electrolytes. For example, a 

 solution of argentic chloride, on adding a slight excess of soluble argen- 

 tic salt, becomes opalescent far more quickly in the presence of much 

 sodic nitrate or nitric acid than in the absence of such an electrolyte. 

 At the end of five minutes the former solution appears about three times 

 as cloudy as the latter ; and several hours are needed for the two solu- 

 tions to attain equality of opalescence at a maximum value, which there- 

 after remains constant. It appears from this fact that the precipitate 

 probably at first forms in a colloidal s'tate, since electrolytes are known 

 to hasten the precipitation of colloids. This inference is confirmed by 

 the occasional appearance of pale blue or pink tints in the incipient 

 precipitate, which disappears as the cloudiness becomes more intense. 



Oddly enough, a solution of pure argentic chloride (made by shak- 

 ing the curd)' precipitate with water and filtering) attains its maximum 

 cloudiness after subsequent addition of argentic nitrate more rapidly 

 than a solution of similar dilution made by mixing equivalent amounts 

 of very dilute argentic nitrate and sodic chloride solutions. The trace 

 of sodic nitrate left in the latter case seems to be unable to counteract 

 some other tendency holding the precipitate in solution. At first it 

 seemed possible that minute invisible ' ' crystal germs ' ' might be 

 present in the filtered solution, which accelerated the appearance of 

 the opalescence, but this surmise was soon overthrown'experimentally. 

 A solution made from curdy silver chloride and found nephelomet- 

 rically to contain 1.28 milligrams per liter, was diluted with an equal 

 bulk of pure water, and its concentration after agitation for over one 

 hour was determined by the nephelometer. The average concentra- 

 tion found by five trials, 0.61 milligram per liter, agreed with that 

 expected within a reasonable limit of error, and moreover the speed 

 of precipitation was no less than before. There could hardly have 



