1146 
MR. a. F. RODWELL ON THE EFFECTS OF HEAT ON CERTAIN 
The specific gravity of the iodide in the molten condition is 5'6247. 
The fact that a substance may possess two or three different coefficients of expansion 
has apparently only been observed hitherto in the case of such substances as fusible 
alloy, because in determining the coefficients of solid bodies temperatures exceeding 
100° C. have rarely been employed. Paraffine or ceresineif used as a heating medium 
will allow the determination of coefficients to a temperature of 340° C., and, un¬ 
doubtedly, many bodies if thus examined would be found to present anomalies similar 
to those remarked in the case of the iodides of lead and mercury. 
9. The Lead-Silver Iodide. 
Bearing in mind the peculiar nature of the coefficients of the chlorobromiodides 
of silver [vide p. 1140), it was thought to be advisable to determine the coefficients of 
a lead-silver iodide. 
These bodies were accordingly fused together in the proportion of one molecule of 
each, viz.: PbI 3 .AgI. This contains in 100 parts 
Iodide of lead.=66-206 Lead.=29-7449 
Iodide of silver.=33-794 Silver.=15-5642 
Iodine.=54-6909 
100-000 100-0000 
The substances were fused together in a porcelain crucible, and cast in thin glass 
tubes 9 inches long by one-third of an inch in diameter. The molten mass underwent 
the same changes of colour in cooling as either one of its constituents, and ultimately 
became a dull orange-coloured compact mass. Although composed of two substances 
which are highly crystalline and brittle, the alloy was found to be hard and tenacious. 
Although the constituents are coarsely crystalline in structure, the alloy is finely 
granular. During the cooling of the mass it expanded with sufficient force to break 
the glass tube. Harsh, noises were emitted during cooling, and the whole mass was 
sometimes jerked from its position; while, if held in the hand, it was felt to be 
agitated by strong tremors. 
Mr. T. Carnelley has determined for me the melting point of the alloy, which he 
finds to be 350° C. 
The specific gravity is 5'923. 
By repeated digestion with large volumes of boiling water the alloy is decomposed, 
the iodide of lead being dissolved, while the iodide of silver remains as a dull green 
powder. 
On examination in the expansion apparatus the alloy was found to undergo slow 
expansion to a temperature of 118° C., then, for 6° O., it simply absorbed heat without 
either contracting or expanding. At 124° C. contraction commenced, and continued 
