286 



G. F. Rodwell and H. M. Elder on [Jan. 30 P 



volatilisation of the iodide at a temperature slightly exceeding its 

 melting point, but the rod when cold was found to be so brittle that 

 it usually broke*in the attempt to remove the glass envelope from the 

 outside. Eventually good rods were procured by slowly melting the 

 iodide in thin glass tubes and annealing in hot paraffine. When the 

 whole was cold the glass was cut on the outside, and carefully broken 

 off the ends of the rod, which were sawed plane by a fine steel saw, 

 and then furnished with metal caps, and the rod was placed between 

 the levers of the expansion apparatus. After heating the bar once or 

 twice in paraffine to a temperature approaching its melting point, 

 longitudinal rifts appeared in the glass envelope, which was then 

 easily removed, leaving a clean homogeneous rod of the iodide. 



On heating a mass of the crimson amorphous iodide, it turns yellow 

 at 126° C, and just before the melting point is attained the yellow 

 changes to a deep red-brown. The liquid resulting from the fusion 

 has the appearance of liquid iodide of silver, that is to say, it has the 

 exact colour of bromine. The liquid when cooled solidifies to a red- 

 brown solid which speedily becomes yellow, and at 126° C. it changes 

 to the crimson octohedral variety. Distinct cracking sounds, due to 

 inter-molecular movements, were heard during the continuance of the 

 change. Heat is absorbed when the red iodide changes to yellow, and 

 is given out when the yellow iodide changes to the red. 



A bar of the iodide was placed in the expansion apparatus, melted 

 paraffine was poured upon it, and when the index had become quite 

 steady, a gentle heat was applied to the paraffine. The index showed 

 a regular and slow expansion until a temperature of 126° C. was 

 reached, when the bar began to change from the octohedral to the 

 prismatic condition, and without further rise of temperature rapid 

 expansion took place. The temperature was kept constant until the 

 change was complete, and was then slowly raised. A regular expan- 

 sion now took place under a higher coefficient than before the mole- 

 cular change, and this continued until the melting point was attained. 

 The results were concordant. 



The expansion in passing from the solid to the liquid condition was 

 determined by weighing mercury in a tube, and afterwards filling it 

 to the same height with fused iodide. The specific gravity of each 

 substance being known, and the weight of equal volumes, the expan- 

 sion could obviously be readily determined. 



The coefficient of cubical expansion for 1° 0. from 0° C. to the 

 point of change — 126° C. — was found to be : — 



•0000344706. 



At 126° C, during the change from the red octohedral to the yellow 

 prismatic condition, the body increased in bulk to the extent of : — 



•00720407. 



