ii44 MR. G. F. RODWELL OH THE EFFECTS OF HEAT ON CERTAIN 
8. Di-iodide of Lead, Pbl 2 . 
Pure iodide of lead was cast into rods one-tliird of an inch in diameter and 6 inches 
in lengtli. The ends were made plane by a fine steel saw, and tliey were furnished 
with copper caps. The iodide underwent the same changes of colour as were observed 
in the iodide of silver; that is to say, it fused to a bromine-red liquid, which, when 
solidified, became red-brown, and, while cooling, brick-red, reddish-yellow, and, when 
completely cool, orange-yellow. Harsh noises, like those produced by bending tin, 
were heard during the cooling of the mass, and the fracture was highly crystalline. 
Differences of opinion exist as to the effect of fusing iodide of lead in the air. 
Experiments showed that it may be fused out of contact with air, with scarcely 
appreciable loss. When, however, it was fused in contact with air, rapid decomposition 
ensued. 
When the iodide was heated in a current of carbonic anhydride, it sublimed un¬ 
changed in crystals; while if it was heated in a current of dry oxygen it rapidly 
decomposed, fine crystals of iodine collecting in the fore part of the tube. 
The specific gravity of iodide of lead, in common with the iodides of copper, silver, 
and potassium, is less than the mean specific gravity of its constituents. Karsten 
found it to be 6‘0282, Boullay 6T1, and my own determinations gave 6T2. The 
calculated specific gravity is 6'629. 
The fusing point as determined by Mr. Carnelley is 383° C. 
The coefficient of cubical expansion for 1° C. was found to be 
•00007614 
for temperatures between 0° C. and 205° C. It increased to 
*00008317 
between 205° O. and 253° C. 
Between 253° C. and 265° C. the mass expanded rapidly, with a coefficient nearly 
eight times greater than the previous, viz.:— 
•0006378. 
After the subsidence of this rapid expansion it no longer retained the original co¬ 
efficient, but assumed one of more than double the amount, viz.:— 
•000180. 
At temperatures some distance from the melting point the rod began to bend, and it 
became necessary to assume that this last coefficient continues to the melting point. 
The expansion in passing from the solid to the liquid condition was determined by 
the method previously described. 
It will be observed that the iodide of lead, as in the case of the iodide of mercury 
(vide p. 1142), has three coefficients of expansion, viz.:— (a) a coefficient somewhat less 
than that of chloride of silver up to 253° C.; (b) a coefficient during 12°C., nearly 
