SOME coNTF.Mi'i>R.ti<:y .iin\i.\cr.s i.\ finsiis r t\.v 



prisingly little ovi-r a \\\t\v r.inni- of mrt.ils al r()uin-u-miM.-r.itiirf .iml 

 over a fairly wide r-in^e of leniperatures for each of luaiu' m»'l.;J>. 

 It is usually some\vh.i( larger lliaii tiie predirled \alue (o); hut this 

 can he convenieiilly explained hy siiying tiiat there uuisl he an addi- 

 tional mechanism for transmiltins; heat, something; in tlie nature of 

 the elasticity of the sulistanre, which superposes ils conductinf;- 

 pf>wer ufKin the conduct ing-power of the electrons, and so inflates 

 the numerator of the ratio in (.i). The reason for supposing such an 

 extra mechanism is primarily that there must he some such mechan- 

 ism to perform the thermal conduction in suhstances which are 

 electrical insulators. \o element conducts heat as hadly as sulphur 

 and horon conduct electricity; and if we imagine a special elastic 

 mechanism for conducting heat in boron and sulphur, we can hardly 

 deny it to copper and silver. Bridgman found that for six metals 

 out of eleven tested, the thermal conducti\-ity decreased when high 

 pressure was applied, although the electrical conductivity incieased. 

 We must hope to find an explanation for this anomaly in the hehaviour 

 of the elastic mechanism; likewise an explanation for the de\iations 

 from (5) which occur at high and at low temperatures. In theories 

 such as the one mentioned over Wien's name in the last paragraph, 

 in which the average vis viva of the electrons is supposed not to vary 

 from a hotter place in a metal to a cooler place, we have to lay the 

 entire burden of thermal conduction upon the elastic mechanism. 

 This makes it difficult to explain the universal relation (5). 



Another striking feature of the theory is that Lorentz succeeded 

 in deducing the Rayleigh-Jeans radiation-law from it. He obtaine<l 

 from it an expression for E, the radiant emissivity of a thin stratum 

 of metal, as a function of temperature 7' of the metal and wavelength 

 X of the radiation; another for .1, the absorbing-power of the metal, 

 likewise a function of T and X; divided the first by the second, 

 and obtained a definite quotient. By Kirchhoff's thermodynamic 

 laws, E A is equal to £,., the radiant emissivity of a perfectly black 

 bo<ly. The expression deduced by Rayleigh and by Jeans for £o 

 and the expression deduced by Lorentz for E/A are identical. Lorentz 

 assumetl that the collisions of the electrons with the atoms (or what- 

 ever it is they collide with) are very short in duration compared with 

 the intervals of free unaccelerated flight from one collision to the 

 next, and that the speeds of the electrons are distributed according 

 to Maxwell's law about the mean value corresponding to the mean 

 energy likT. 2. He also made certain assumptions which restrict 

 the validity of his expression for E/A to radiations of great wave- 

 length; the Rayleigh-Jeans expression for E., is restricted in exactly 



