On the Coefficients of Expansion, Sfc. 



545 



It will be observed that the iodide of lead, as in the case of the 

 iodide of mercury (" Proc. Roy. Soc," vol. 28, p. 284), has three 

 coefficients of expansion, viz. : — (a) a coefficient somewhat less than 

 that of chloride of silver up to 253° C. ; (&) a coefficient during 

 12° C, nearly eight times greater than the preceding ; and (c) finally 

 a coefficient somewhat more than twice as great as that between 0° 

 and 253° C, at temperatures above 265° C. Undoubtedly the iodide 

 of lead, as in the case of the iodide of mercury, undergoes a molecular 

 change while rapidly expanding between 253° and 265° C, and before 

 assuming^ the higher coefficient. This is supported by the fact that 

 the highly brittle and crystalline rod showed itself capable of bending 

 after having undergone the rapid expansion. It will be remembered 

 that the iodide of silver, which is very crystalline and brittle below 

 145° C, becomes amorphous and plastic above that temperature. 

 The familiar example of sulphur will also recur to the mind. 



If we suppose a mass of iodide of lead to be heated from 0° C. to 

 the melting point (383° C.) the following will be the volumes at the 

 respective temperatures. 



Volume at 0° C. =1*000000. 



205° =1-015608. 

 253° =1-019595. 

 265° =1-027248. 

 383° (solid) =1-048488. 

 383 c (liquid) =1-078080. 



The curve is shown in Table A. 



The specific gravity of the iodide in the molten condition is 5*6247. 



The fact that a substance may possess two or three different co- 

 efficients 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° 0. have 

 rarely been employed. Paraffine or ceresine used as a heating medium 

 will allow the determination of coefficients to a temperature of 

 300° C, and, undoubtedly, many bodies when thus examined would 

 be found to present anomalies similar to those remarked in the case of 

 the iodides of lead and mercury. 



The Lead- Silver Iodide Alloy. 



Bearing in mind the peculiar nature of the coefficients of certain 

 alloys of iodide of silver with the chloride and bromide (" Proc. Roy. 

 Soc." vol. 25, p. 292), it was thought to be advisable to determine 

 the coefficients of an alloy of iodide of lead with iodide of silver. 



These bodies were accordingly fused together in the proportion of 

 one molecule of each, viz., PbL.Agl. This contains in 100 parts 



