BAXTER. — A REVISION OP THE ATOMIC WEIGHT OF IODINE. 423 



equal volume of water and was heated until all nitrous acid was destroyed 

 and all oxides of nitrogen were expelled. It was then transferred to one 

 of the precipitating flasks. These were Erlenmeyer flasks of one and 

 two liters capacity, into the necks of which glass stoppers had been 

 carefully ground.* Into the solution of silver nitrate, which had been 

 diluted until not more than one gram of silver was contained in one 

 hundred cubic centimeters, and to which an excess of ammonia had 

 been added, a slight excess of the solution of pure ammonium iodide, 

 which also had been diluted until it was not stronger than one per 

 cent, was poured. The flask was stoppered and shaken for some time 

 and was allowed to stand for about a day. Then a very slight excess of 

 nitric acid was introduced, the flask was again shaken for some time, 

 and was allowed to stand for one or two days until the solution above 

 the precipitate seemed perfectly clear. The precipitate was washed by 

 decantation with one per cent nitric acid at least eight times, and was 

 transferred with water to a weighed Gooch crucible through which the 

 washings had been poured. Finally the crucible was heated in an electric 

 air-bath for several hours, first at 100-110°, then at 200° C. 



Although silver iodide is very slightly sensitive to diffused daylight, 

 the operations of precipitation and filtration were performed in a large 

 cupboard lighted with orange light. When taken out of the dark room, 

 the flask was enveloped in several thicknesses of black cloth. 



The tendency of precipitated silver iodide to pass into a milky condition 

 when washed with pure water is very marked. It has been found by 

 Stas and others that if the water is previously heated to about 60° C, 

 and plenty of time is allowed for the precipitate to settle at that tempera- 

 ture, this tendency can be overcome. When washed with warm water, 

 however, silver iodide adheres to the flask to such an extent that it is 

 impossible entirely to remove the precipitate even by vigorous rubbing. 

 The conditions which were found most favorable for preserving the 

 precipitate in such a state that it could be easily and rapidly manipulated 

 were as follows. In the first place precipitation was carried out in 

 ammouiacal solution, because the silver iodide clotted together much 

 more rapidly than in acid solution, owing probably to the greater solu- 

 bility of silver iodide in the alkaline solution. A large excess of nitric 

 acid also coagulated the precipitate, but since the acid caused the separa- 

 tion of a large amount of iodine, it could not be employed in the present 

 case. Dilute nitric acid, on the other hand, proved to be a satisfactory 



Kicliards, Proc. Amer. Acad., 28, 24. 



