1142 
MR,. G. F. RODWELL ON THE EFFECTS OF HEAT ON CERTAIN 
The vapour condenses to rhombic prismatic crystals, which frequently become scarlet 
while cooling, but which, if they still remain yellow when cold, instantly become 
scarlet if rubbed or otherwise mechanically agitated. 
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 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 pris¬ 
matic 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 expansion now took place under a higher coefficient than 
before the molecular change, and this continued until the melting point was attained. 
The coefficient of cubical expansion for 1° C. 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. 
The coefficient of cubical expansion for 1° 0. from 126° C., after the change to the 
melting point, 200° C., was— 
•0001002953. 
Thus, if we suppose a molten mass of the iodide of mercury to be cooling down from 
200° C. to 0° C., the following would be the volumes under the conditions indicated:— 
Volume at 200° C. of the liquid mass . ..=1-1191147 
„ „ „ solid mass.=1’0190453 
„ 126° C. (yellow prismatic condition).= 1-0115378 
„ „ (red octohedral condition).= 1"0043337 
0 ° 0 .= 1-0000000 
The changes are shown at one view in the accompanying curve table, in which the 
expansion of mercury is given for comparison. 
