On the Coefficients of Expansion, §-c. 



547 



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 sub- 

 stances 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° C, it simply absorbed heat without either contracting or expand- 

 ing. At 124° C. contraction commenced, and continued at unequal 

 rates till a temperature of 139° C. was attained. Then, again, the 

 mass underwent neither contraction nor expansion during heating 

 through 5° C, and then it commenced to expand somewhat rapidly. 

 The most rapid contraction on heating took place between 130° and 

 133° C. Thus, in all, for the temperatures during which the mass con- 

 tracted, the index moved through fifteen revolutions of 180° to 0° of arc, 

 and these were related to the temperatures in the following manner : — 



1 revolution of index took place during heating from 124 — 128° C. 



2 „ „ „ 128—130° C. 

 4 „ „ „ 130—131° C. 

 6 „ „ „ 131—133° C. 

 2 „ „ „ 133—139° C. 



The details of these contractions are shown in Table B. 



The heating, especially at these temperatures, was excessively slow, 

 and so moderated that a complete observation of the behaviour of the 

 substance in the expansion apparatus lasted from three to four hours. 

 Above 144° C. the alloy expanded with a coefficient about three 

 times greater than that which it possessed between 0° and 118° C. 



