192 M. Stas on the Determination of Atomic Weights. 



The boiling solution of this body gives,, with broniate of potas- 

 sium, a broniate of silver which is unchangeable in the light ? 

 and can be boiled with water without decomposition. It soon 

 undergoes decomposition in contact with organic matter (under 

 the air-pump, for example) . Hence all the water used for the 

 decomposition had to be twice distilled over manganate of po- 

 tassium, 



II. 



The second part of the research contains new determinations 

 of the atomic weights of silver, iodine, bromine, and chlorine. 



Iodide of silver. — 1. A weighed quantity of silver (97 and 43 

 grms.) was dissolved in nitric acid of 1*21 to 1*25 spec. grav. 

 The long-necked flask was provided with a bulb apparatus which 

 contained some water, and the other end of which dipped in a 

 small flask also containing water. The bulb apparatus was 

 ground in the neck of the flask; and all parts of the apparatus 

 were separately weighed. The heating was effected in a gas- 

 furnace, in which the heat was made as uniform as possible by 

 several superimposed layers of wire gauze. When all the silver 

 was dissolved, the contents of the bulb apparatus and of the 

 little flask were added to the principal quantity of the liquid, 

 and the liquid distilled off at as low a temperature as possible. 

 The residue was treated with concentrated pure sulphuric acid, 

 and the sulphate of silver formed again heated with sulphuric 

 acid and with sulphate of ammonia. The acid distillate obtained 

 from the evaporation of the solution of silver was evaporated in a 

 porcelain dish, and the solution also treated with sulphuric acid. 

 All the sulphate of silver is now dissolved in water and precipitated 

 by a solution of hydriodic acid. The latter is prepared by the 

 addition of a very dilute solution of sulphurous acid to iodine 

 diffused in ice-cold water. If the solution is not kept cool, or if 

 too much sulphurous acid is used, sulphur is separated in the 

 reaction. The precipitate is well shaken and washed by decan- 

 tation with hot water. After the wash-water has been allowed 

 to stand, it is filtered through a small filter, and the filtrate first 

 concentrated in a glass retort, then in a platinum one, and after- 

 wards in a platinum dish. There remained a residue of only 

 0*6 milligramme, from which 0*6 milligramme of Agl were 

 separated. 



The iodide of silver is introduced into a glass bulb provided 

 on two opposite sides with tubulures, one of which is closed with 

 fine platinum wire. The iodide of silver is dried in this bulb in 

 the air-bath and weighed. In order that while the aqueous 

 vapour is escaping no iodide be lost, both tubulures of the glass 

 bulb are provided with weighed flasks. The condensed aqueous 



