Radiation on the Transmission of Heat. 253 



When t is sufficiently large the reflecting property of the 

 boundary becomes immaterial, and the transmitted heat is 



The ordinary measurements of thermal conductivity neglect 

 the effects of radiation and would yield incorrect results if 

 2R//r\ were appreciable. This will only be the case for 

 transparent bodies possessing very low conductivities. When 

 the measurements are made at the ordinary temperatures, it 

 is the transparency for rays of low refrangibility that comes 

 into play, so that rock-salt and fluorspar are the only bodies 

 which can come into consideration. According to Melloni's 

 experiments the constant k for rock-salt is *32, but this must 

 be doubled to make it apply to rays traversing the substance 

 obliquely as well as normally (see § 7). The thermal con- 

 ductivity of rock-salt according to Lees is *014. The fraction 

 ^R/atX in this case works out to be "002. The measurements 

 of conductivity are affected by an error of that amount, 

 which is probably beyond the reach of experiment, but if rock- 

 salt possessed a thermal conductivity as low as that of cork, 

 or '00013, the error committed in measurements of thermal 

 conductivity by neglecting radiation would amount to 20 per 

 cent. That the effects of radiation may be neglected in pro- 

 portion as the body absorbs the radiations may appear 

 surprising at first sight, but is explained by the fact that in 

 the case of good absorbers any portion of the body is 

 affected only by the radiation coming from other portions in 

 its immediate neighbourhood which possess temperatures 

 nearly equal to its own. 



6. The effects of radiation become conspicuous, and more 

 important than those due to conduction of heat, in the case 

 of gases ; but the ordinary method of treatment which con- 

 siders the two effects separately is, according to § 5, sufficient, 

 when the thickness of the layer is not too large, so that a 

 considerable fraction of the heat-radiation may pass through 

 it. This investigation has been undertaken primarily with a 

 view of clearing up the influence of radiation on the tem- 

 perature distribution inside a large gaseous body, such as we 

 imagine the sun to be. Here we have to deal with a great 

 thickness which ultimately must be opaque. How far very hot 

 gases may be transparent it is difficult to judge, bur opaque- 

 ness must ultimately result, if only from the scattering of 

 light by molecules, assisted probably to a. great extent In- 

 internal reflexions taking place at surfaces at which the 

 density or temperature varies very rapidly. We are all 



