530 Mr. 0. J. Lodge on a Mechanical Illustration 



tion to metals (cf. Professor Ketteler in the November Number 

 of the ' Philosophical Magazine,' p. 333) ; so we will suppose 

 the same to hold good with them. 



Again, the velocity of light in a medium {other things being 

 equal) will be proportional to the square root of the elasticity of 

 the threads joining the buttons (§§8 and 18); for the maxi- 

 mum velocity V of a particle is a >J k (§ 22); and this is related 

 to the velocity of propagation of light L by the equation 



V:L = 27m:\, 



X being of course the wave-length, whence 



But the velocity of light in a medium is known to be inversely 

 as fjb : therefore 



u? oc- ocK, . (6) 



K 



(see § 8), which appears to agree with Maxwell's theory that 

 the index of refraction of a substance is nearly equal to the 

 square root of its specific inductive capacity K. 



We have found, then, that the internal radiating-power, 

 which we have shown to be proportional to r, is also inversely 

 proportional to tc, because directly proportional to y? ; hence 



tcozr- 1 ; (7) 



or conductivity for heat and electric conductivity in metals are 

 proportional to one another (for we saw in § 18 that k was 

 probably proportional to the conductivity for heat); and there- 

 fore the conductivity for heat varies with the temperature ac- 

 cording to the law 



~f-ra. < 8 > 



which is a curve not very different from -~. It vanishes for 



6= cc , and it becomes great and equal to y-p — for = 0. 



The law of the steady conduction of heat parallel to the axis 

 of x through a slab is 



at ax 



Now Professor Tait, in his ' Recent Advances in Physical Science,' 

 p. 271, says that when everything has attained a permanent con- 



