540 



Mr. G. F. Rodwell. 



packed. Tliere is a different arrangement for holding the minor weight, which the 

 engraver has failed in representing so well as the rest. 



Plate 9 shows also a variety of milled screw-heads, each of which has of course a 

 purpose ; but as neither of them, except one by which the lower weight is turned 

 round, has any part in the observations such as they are described above, but only in 

 ultimate manipulations which it is unnecessary to dwell upon here, they may be 

 regarded as ornaments. It will be noticed that the collimator is removed, and one 

 of the side supports is supposed broken off, to discover the minor weight ; which 

 last, ^with its inirror and magnet bar, is seen turned through 30° or 40° into an 

 oblique position. At the same level and outside the chamber are seen arms, one 

 of which resembles a cross. These are the guides for two magnets which have 

 been removed. Their intended purpose is mentioned by Broun in his description. 



The pillar alongside the shaft is a case for a thermometer, the bulb of which is 

 within the chamber. 



" On the Coefficients of Expansion of the Di-iodide of Lead, 

 Pbl 2 , and of an Alloy of Iodide of Lead with Iodide of 

 Silver, PbL.Agl." By G. F. Rodwell, F.R.A.S., F.C.S., 

 Science Master in Marlborough College. Communicated 

 by Professor A. W. Williamson, For. Sec. R.S. Received 

 March 10. Read March 31, 1881. 



In former communications which I have had the honour of sub- 

 mitting to the Royal Society, I have given determinations of the 

 coefficients of expansion by beat of the chloride and bromide of silver 

 and the iodide of mercury between 0° C. and the fusing point ; also 

 determinations of the coefficients of expansion and contraction of the 

 iodide of silver, and of certain chlorobromiodides of silver. (" Proc. 

 Roy. Soc," vol. 25, pp. 280-303, and vol. 28, p. 284.) 



The iodide of lead, and an alloy of iodide of lead with iodide of 

 silver, were thought to be very suitable substances for a continuation 

 of these experiments. The following pages describe the results 

 obtained. 



The experimental method was precisely similar to that before 

 described, but the expansion apparatus was rendered more delicate by 

 several notable changes suggested during the course of the former 

 experiments. It is unnecessary to describe this apparatus again (for 

 description vide " Proc. Roy. Soc," vol. 25, p. 281-2), but it may be 

 remembered that a homogeneous rod of the substance under examina- 

 tion is connected with a series of levers which multiply 5,382 times, 

 while the value of the movements is estimated by a micrometer screw 

 reading to 50 5 00 of an inch. The following alterations were made 

 mainly with a view of reducing the resistance by diminishing fric- 

 tion, and thus adding to the sensibility of the apparatus : — 



1. The wooden base N (fig. 1) was replaced by a massive stone 



