M. Stas on the Determination of Atomic Weights, 193 



vapour is expelled and the flasks again weighed. The iodide of 

 silver is first weighed at 100°, then at 200°, and is finally fused. 

 The weight remained unaltered. During the whole operation the 

 iodide of silver remained in the dark. Pure iodide of silver is of 

 a dirty bright yellow, and is unaltered in the light. It is only 

 altered by light in the presence of sulphurous acid. Fused iodide 

 of silver varies from yellow to dark reddish brown. It attacks glass 

 more slowly than chloride or iodide of silver. Treated in this 

 manner, 100 parts of silver gave in the mean 217*5325 parts of 

 iodide of silver; the mean difference was + 00035. The iodine 

 used for the determinations of the atomic weights was purified 

 in two ways. Eight pounds of iodine were dissolved in a con- 

 centrated solution of two pounds of iodide of potassium, and then 

 not more than three-fourths of the iodine precipitated by water. 

 The washed precipitate was distilled off in a current of aqueous 

 vapour, the distillate dried over nitrate of calcium (this being the 

 only substance that can be used), and, in order to remove all 

 water and HI, distilled twice over anhydrous caustic baryta. 

 Or one pound of iodine is agitated with highly concentrated am- 

 monia, and the almost colourless liquid separated by a drawn-out 

 funnel from the iodide of nitrogen formed. It is washed by de- 

 cantation with concentrated ammonia, and at length upon the 

 filter with cold water. The iodide of nitrogen, while still moist, 

 can be rubbed in a mortar without danger. Diffused in much 

 water, it slowly decomposes at the ordinary temperature, and ra- 

 pidly and without danger between 60° and 65°, pure iodine being 

 separated. Iodide of ammonium remains in solution along 

 with another salt, which is white and explosive and very little 

 soluble in cold, but more easily so in hot water (iodate of ammo- 

 nium). The washed iodine is distilled with aqueous vapour, in 

 which the admixed salt remains behind. The latter decomposes 

 at a little over 100° with a slight explosion, and separates iodine 

 and gases. The separated iodine is purified as above. The 

 quantity of iodine obtained from the iodide of nitrogen was 

 always less than it ought to be from theory, if iodide of nitrogen 

 had to decompose into iodide of ammonium, iodine, and ni- 

 trogen. 



Pure iodine differs in appearance from commercial. Fused, it 

 is fluid, and almost entirely black. At ordinary temperatures it 

 emits no vapours. The saturated vapour is intensely blue, and 

 violet in the diffused state. Iodine does not melt at 113°, but 

 is liquid at 115°. The boiling-point is above 200°. Ordinarily 

 the fusing-point is stated to be 107°, and the boiling-point 

 175°-180°. 



2. The complete synthesis of iodide of silver was effected in two 

 ways, (a) Pure iodine was sublimed in a glass bulb, rubbed while 



Phil Mag. S. 4. Vol. 33. No. 222. March 1867. 



