24 



On the Coefficients of Expansion, fyc. [Mar. 31, 



bromidides of silver, continues the same methods of determining the 

 coefficients of the iodide of lead, and of an alloy of the iodides of lead 

 and silver. Various alterations in the apparatus, which by the 

 diminution of friction and other means have rendered, it much more 

 delicate than before, are described. The iodide of lead was found to 

 possess three coefficients of expansion; the first for temperatures 

 between 0° and 205°C.— 



•00008317 cubical expansion for 1°C. ; 



the second, a very high coefficient, between 205° C. and 253° C. — 



•0006378 ; 



and the third, for temperatures between 253°C. and the fusing 

 point — 



•000180. 



The volumes at the different temperatures are given and tabulated, 

 and the curve of expansion is plotted. 



The lead silver iodide, PbI 2 .AgI, is next examined. 



It 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 



Between 0° and 118° C, it slowly expands when heated, with a 

 cubical coefficient for 1° C. of 



•0000306. 



Then for a few degrees (118 — 124° C), it simply absorbs heat 

 without contracting or expanding. On reaching a temperature of 

 124° C, the mass commences to contract on further heating, and this 

 continues until a temperature of 139° C. is attained. Details of the 

 contraction are given, and the curve of contraction is compared with 

 that of the iodide of silver. Between 139 and 144° C, heat is again 

 absorbed without change of volume ; and above 144° C. the alloy 

 expands somew r hat rapidly, with a coefficient of 



•0001150. 



The volumes at various temperatures between 0° and the fusing point 

 are given, and are shown in a curve. 



The following points are noted in regard to the alloy : — 



1. It possesses a similar density at three different temperatures, as 

 at 0° C, 130° C, and 282° C. 



2. Although it contains only 33*794 per cent, of iodide of silver, it 

 contracts as considerably during heating as the iodide of silver itself. 



