THE RATIO OF SODIC TO ARGENTIC CHLORIDE. 37 



permanent occlusion of sodic and argentic nitrate in the manner already 

 stated, but too great promptness prevents complete precipitation of the 

 silver halide. About fifteen hours was usually allowed for the settling 

 of the solution. Agitation is not necessary. 



The clear supernatant liquid was decanted through a large, care- 

 fully prepared Gooch crucible. The mat of this Gooch filter, through 

 which all the mother liquor and wash waters were passed, was prepared 

 of selected fibrous asbestos shreds, which had been boiled with nitric 

 acid and well washed. The mat was a trifle thicker than a filter 

 paper so that the holes of the crucible were indistinctly visible, when 

 viewed in front of a strong light and was not allowed to extend up 

 the sides of the crucible. A preliminary ignition of the mat was found 

 to result in a slight disintegration and brown coloration, and caused 

 loss during washing ; therefore before weighing the mat was dried at a 

 lower temperature, namely, about 150 . This same temperature was 

 used later in drying the argentic chloride. During the operation of 

 washing at first all but a trace of the precipitate was left in the flask, 

 and it was then rewashed by decantation. After each washing, the 

 precipitate was allowed to drain several minutes, for silver chloride is 

 known to hold water like a sponge. At the second or third washing 

 a disintegration of the precipitate often took place, resulting in an 

 emulsionlike mixture, but in most cases this could be prevented by the 

 presence of a little nitric acid. The precipitation had been made with 

 silver nitrate in excess. The first three washings of 0.2 liter each 

 were made with very dilute silver nitrate, and this was displaced in the 

 fourth washing by about 0.05 liter of pure water. In such wash 

 waters, which, with the mother liquor, usually amounted to about 2 

 liters, no trace of chloride could be found by the nephelometer. The 

 main weight of silver chloride was corrected by + 0.06 milligram, 

 however, in every case, for the otherwise determined solubility in such 

 wash waters (p. 30). From five to eight subsequent treatments 

 with pure water, very slightly acid, in 0.1 liter portions, completed 

 the washing. These later wash waters were treated in the manner 

 described below, in order to determine the chloride which they always 

 contained. 



The precipitate was now transferred to the filtering crucible wholly 

 by means of a stream of water. It was found convenient first to 

 transfer and pack down by air pressure a small portion of the precipi- 

 tate on the asbestos mat, in order that the greater part of the argentic 

 chloride might be easily separated from this mat for fusion. By resting 

 the lip of the flask on the Gooch crucible it is possible to direct a 

 stream of water from a wash bottle into the flask and finally wash out 



