552 On the Coefficients of Expansion, fyc. 



The accompanying tables show the curves of contraction and ex- 

 pansion of the lead-silver iodide alloy, and of its constituents. In 

 Table B the scale has been enlarged in order to show the details of 

 the contraction of the alloy on being heated from 124° C. to 139° C. 



(Addendum. Received April 8, 1881.) 



Other alloys of iodide of lead with iodide of silver have since been 

 made, having the following composition : — ■ 



(1.) 2AgI.PbI , containing 50*517 per cent, of iodide of silver. 

 (2.) 3AgI.2PbI 2 „ 43-360 

 (3.) 4AgI.3PbI 2 „ 40-497 

 (4.) 5AgI.4PbI 2 „ 38-950 

 (5.) 10AgI.9PbI 3 „ 36-190 



These all possessed the same general appearance as the alloy 

 AgT.PbI 2 described above, which contains 33*794 per cent, of iodide 

 of silver. But with the exception of No. 5 they were all so brittle 

 that they conld not be cast into rods suitable for nse with the expan- 

 sion apparatus. During cooling large rifts appeared in the rod at 

 right angles to its length, at the time when the iodide of silver com- 

 menced to expand. In the case of No. 1 the rod was violently broken 

 during its cooling by the expanding iodide of silver ; even when 

 slowly annealed in hot paraffine. It may be noted that no such effect 

 was produced in the case of the chlorobromiodides of silver, having 

 the composition respectively: Ag 2 I 2 .AgBr.AgCl ; Ag 3 I 3 .AgBr.AgCl ; 

 Ag_J 4 . AgBr. AgCl ; and containing in each case a larger percentage of 

 iodide of silver (viz., 58*6404; 68-0171; and 73-9285) than the 

 silver-lead iodide alloy No. 1. The chlorobromiodides, although, of 

 course, their brittleness increased with the percentage of iodide of 

 silver, formed less brittle rods than the iodide of silver, and than the 

 first of the silver-lead iodide alloys, although the latter contains 

 23 per cent, less iodide of silver than the chlorobromiodide 

 Ag,I 4 .AgBr.AgCl. 



