94 RESEARCHES UPON ATOMIC WEIGHTS. 



a platinum condenser. The water was distilled twice, once from alkaline per- 

 manganate, once from a solution containing a trace of sulphuric acid. In both 

 distillations block tin condensers were used. 



METHOD OF SYNTHESIS. 



In order to convert a known weight of silver into silver iodide essentially 

 the same method was used as that employed in the research upon the synthesis 

 of silver bromide (page 57). The silver was dissolved slowly in redistilled 

 nitric acid which had been diluted with an equal volume of water in the flask 

 described on page 12. The solution was diluted and was heated until all 

 nitrous acid was destroyed and all oxides of nitrogen were expelled. It was 

 then transferred to a glass-stoppered precipitating-flask and after dilution 

 until not more than i gm. of silver was contained in 100 c.c, an excess of am- 

 monia was added and then a slight excess of the solution of pure ammonium 

 iodide, which also had been diluted until not more concentrated than i per 

 cent. The flask w^as stoppered and shaken for some time, and was allowed to 

 stand for about a day. Next 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 i 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° to 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 colloidal 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 temperature, 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 precipi- 

 tate even by vigorous rubbing. 



The conditions 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 ammoniacal solution, because the silver 

 iodide clotted together much more rapidly than in acid solution, owing prob- 

 ably to the greater solubility of silver iodide in the ammoniacal solution. A 

 large excess of nitric acid also coagulated the precipitate, but since the acid 

 caused the separation 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 satis- 



