248 HEAT. 



doubt that the residual gas played some part not eliminated, and that 

 the radiation is not truly represented by the formula. 



Rosetti's Law. Rosetti* found that with a Leslie cube or an iron 

 vessel filled with mercury, but not surrounded by a lampblack enclosure, 

 the cooling between 300 C. and 0. was much more nearly represented 



by 



when T is the absolute temperature of the body, 6 that of the enclosure, 

 and a and b are constants for the body. 



Stefan's Law. In 1879 Stefan f gave an entirely new turn to the 

 researches on radiation by the suggestion that the rate of radiation is 

 proportional to the fourth power of the absolute temperature. He was led 

 to this by observing that a result of Tyndall's accorded with it. Tyndall 

 found that the radiation from a platinum wire at 1200 0. was 11*7 



/1200 4- 273\* 

 times its radiation at 525 0. Now ( -=^= ===- ) is very nearly 1 1 -7, 



\ t)ZO + 2tlO J 



or the two radiations were almost exactly in proportion to the fourth 

 powers of the absolute temperatures. Re-examining the work of 

 Dulong and Petit and their successors, Stefan found that on taking con- 

 duction by the surrounding gas into account, the radiation was more in 

 accordance with the fourth-power law than with the formulae previously 

 used. In 1875 Bartoli applied thermodynamic reasoning to radiation, 

 and in 1884 Boltzmann followed up Bartoli's work. Starting with the 

 supposition, now proved, J that radiation presses against any surface on 

 which it falls, Boltzmann treated the radiation in an enclosure as the 

 working substance in a Carnot cycle, and showed that the full radiation 

 at any temperature must, if the method is justifiable, be proportional to 

 the fourth power of the absolute temperature. We shall give an account 

 of this thermodynamic treatment in chap, xx Here it is sufficient to 

 say that the subject lias been developed by Wien and others, and that 

 Wien has shown that if a body is emitting full radiation then the wave- 

 length of maximum radiation is given by 



A^fl = constant, 



where 6 is the absolute temperature of the surface, and that the energy 

 radiated through a small range dX at this maximum is 



E TO c? A. = constant x 6 5 . 



Wien on certain assumptions deduced a formula to express the 

 energy radiated for every wave-length. But the results of experiment 

 did not confirm Wien's formula so well as a modification obtained by 

 Planck.|| 



* Phil. Mag., viii., 1879. 



t Wien. Alcad. Ber., 1879, Ixxix, pp. 391-428. 

 $ Lebedew, Congres International de Physique, ii. p. 133. 



Wien, Congrts International de Physique, ii. p. 23 ; Larmor, " Radiation," 

 Encyclopedia Britannica, xxxiii., 10th ed. 



|| Deutsch. Phys. Gesell. Verh., 2, xiii. p. 202; 2, xvii., p. 237, 1900. 



