1881.] Permanent Molecular Torsion of Conducting Wires. 25 



3. While the iodide of silver commences its contraction at 142° C, 

 and finishes it at 145°*5 C, the alloy commences to contract 18° C. 

 lower (viz., at 124° C.) and finishes 6°-5 C. lower (viz., a,t 139° F.). 



4. The chlorobromiodides of silver (" Proc. Roy. Soc," vol. 25, 

 p. 292) also began to contract on heating (an effect which, of course, 

 we must attribute solely to the presence of iodide of silver), at 124° C, 

 "but they finished at 133° C 



5. The harsh sounds emitted by the alloy during cooling, and the 

 tremors simultaneously propagated through the mass, prove that 

 violent molecular agitation is going on at such time as the iodide of 

 silver is passing from the amorphous plastic condition to the brittle 

 crystalline condition, within the mass of the iodide of lead. 



6. The fusing point of the alloy is 125° C. lower than that of the 

 iodide of silver, which constitutes one-third of its weight, while it is 

 only 19° C. higher than that of the iodide of lead, which constitutes 

 two- thirds of its weight. 



7. If the lowering of the fusing point (also markedly apparent in 

 the case of the chlorobromiodides of silver) is due to the fact that 

 similar particles of matter attract each other more powerfully than 

 dissimilar, and hence when the particles of two bodies are mutually 

 diffused, the attraction becomes less, and the molecular motion is 

 consequently more readily assimilated, the same cause may serve to 

 explain the commencement of the phase of contraction on heating the 

 alloy at a temperature of 18° C. lower than the substance to which it 

 owes this property. 



The lead silver iodide alloy is finally compared with a chlorobrom- 

 iodide of silver, which latter, although it contains 8 per cent, more of 

 iodide of silver than the lead silver iodide alloy, undergoes a contraction 

 on heating, which is more than twenty times less, although in both 

 cases we must regard the effect as solely due to the iodide of silver. 



II. "Permanent Molecular Torsion of Conducting Wires pro- 

 duced by the Passage of an Electric Current." By Pro- 

 fessor D. E. Hughes, F.R.S. Received March 17, 1881. 



In a paper on " Molecular Electro-Magnetic Induction," presented 

 to the Royal Society March 7, 1881 (p. 524), I gave a description of 

 the induction currents produced by the torsion of an iron wire, and 

 the method by which they are rendered evident. The electro-magnetic 

 induction balance there described is so remarkably sensitive to the 

 slightest internal strain in anywise submitted to it, that I at once 

 perceived that the instrument could not only determine any me- 

 chanical strain such as torsion or longitudinal stress, but that it might 

 indicate the nature and cause of internal strains. Upon putting the 



