116 Sir W. Thomson on Cauchy's and Green's 



the temperatures of the outside surface and of the inside will 

 after a time become more or less considerable. The outside, 

 being always cooler than the inside, will sooner reach the 

 critical temperature at which there is a sudden increase 

 of thermal conductivity, and, as a consequence, there will be 

 a sudden rush of heat from the inside to the outside ; hence 

 the reglow. The assumption that a sudden increase of thermal 

 conductivity should take place when the metal has cooled to a 

 certain temperature is very reasonable ; for there is, without 

 doubt, a sudden increase (or, to speak more correctly, more 

 than one* sudden increase) of electrical conductivity as the 

 iron cools. 



Mr. H. F. Newall asserts f, however, that he has shown that 

 the reglow is " not due to differences in conductivity in iron 

 at different temperatures,'" and that " there is a rise of tem- 

 perature not only at the surface . . . but also throughout the 

 mass/' Mr. Newall has not yet brought before us any ex- 

 perimental evidence in proof of these assertions ; but he has 

 promised to do so. 



Mr. Newall seems to agree with the author that the process 

 going on during recalescence " partakes of the nature of an 

 explosion, in that once started it continues throughout the 

 mass of the iron/' but he regards the rise of temperature as 

 being caused by internal chemical action. The author looks 

 forward with much interest to the publication of Mr. Newall's 

 results. 



XIII. On Cauchy's and Green's Doctrine of Extraneous Force 

 to explain dynamically FresnePs Kinematics of Double 

 Refraction. By Sir William Thomson.^ 



1. /~^\ REEN'S dynamics of polarization by reflexion, and 

 VX Stokes' dynamics of the diffraction of polarized 

 light, and Stokes' and Rayleigh's dynamics of the blue sky, 

 all agree in, as seems to me, irrefragably, demonstrating 

 Fresnel's original conclusion, that in plane polarized light the 

 line of vibration is perpendicular to the plane of polarization ; 

 the " plane of polarization " being defined as the plane 

 through the ray and perpendicular to the reflecting surface, 

 when light is polarized by reflexion. 



* We should expect, therefore, more than one Burface-reAeateno. 

 t Phil. Mag-, vol. xxiv. No. 150, p. 436 (1887). 



X Communicated by the Author, having been read before the ltu\al 

 Society of Edinburgh, Dec. 5, 1887. 



