THE SOLUBILITY OF ARGENTIC CHLORIDE. 2"J 



in most cases.* This may indeed be the reason why the precipitate 

 appears in a form so loosely knit, for at the moment of precipitation 

 it is almost impossible that the two precipitating liquids should for more 

 than an instant be exactly equivalent. Hence each first particle of 

 precipitate would quickly be surrounded by a liquid unfavorable to 

 its union with another in compact crystalline fashion. 



Mulder thought that an excess of either silver or chloride equiva- 

 lent to four times the weight of the dissolved argentic chloride was 

 enough to complete the precipitation, and Stas thought that exactly 

 three times the equivalent amount sufficed. It is clear now that this 

 limit is determined merely by the efficiency of the means used to 

 detect the last faint cloud of precipitate, and then only approximately. 



As has been said, the changing solubility demands that every 

 filtrate be tested quantitatively. The methods which may be used 

 are all open to some criticism. The determination of electrolytic con- 

 ductivity is in vain when any other electrolyte is present, and its 

 interpretation is complicated even in pure aqueous solution by possible 

 hydrolytic complications. The determination of electromotive force 

 depends upon the rigorous application of the law of concentration 

 effect to electrolytes a somewhat doubtful question. The actual 

 evaporation of the solution and weighing of the precipitate leads to 

 many errors when silver is in excess, f and the determination of the 

 intensity of the opalescence upon precipitating the dissolved chloride 

 depends greatly on the details of the production of the opalescence. 



From among all these methods we chose the last, after a very care- 

 ful study of their probable errors. But although fairly confident of its 

 results, for reasons shortly to be given, we were careful in every case 

 to allow in each experiment as little dissolved argentic chloride as 

 possible, so as to reduce the possible error to a minimum. 



The method of comparing the intensity of the cloudiness of opal- 

 escent solutions and therefore the quantities of suspended precipitate 

 has already been discussed at some length in a recent paper on the 

 nephelometer4 This instrument consists of a device for the compari" 

 son of the tint or brightness of two solutions containing precipitate, 

 viewed against a dark background. By making one of these opales- 

 cent mixtures from a known amount of material, an unknown amount 

 may be estimated by comparison with it. It is easy to show that 



^Compare page 30. 



t Richards, Proc. Am. Acad., 29, 71 (1893). 



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



