508 MR. J. E. PETAVEL ON THE HEAT DISSIPATED BY. 



small, its absolute value has a considerable influence on the rate of cooling of the 



enclosed body. 



SCHLECEKM ACKER'S* results only apply to radiation in a high vacuum, but BOTTOMLEY, 

 in his researches on the same subject, incidentally gives one value at atmospheric 

 pressure ; this is for a temperature of 408 C. It works out at nearly one and a-half 

 times the emissivity for the same temperature as shown in Curve I., fig. 7. 



From the resistance given in Dr. BOTTOMLEY'S paper, it is probable that the wire 

 used was about '3 millim. in diameter. If this be the case, the difference is what 

 might have been predicted from the results obtained by Professors AYETON and 



KlLGOUK.t 



The formulae given by them for the connection between the emissivity of the wire 

 and its diameter are 



At 200 C. e= -00111 + -014303d- 1 . 

 At 300 C. e = '001135 + 016084C?- 1 . 



The wires were heated in a horizontal position and the diameters measured in 

 millimetres. 



The diameter of the wire used was 44 millims. Inserting this value for d in the 

 above formulae gives for the value of the emissivity at 200 and 300 C. "00144 

 and -00150 C.G.S. units- The values given in Curve 1., fig. 7, are '00107 and '00121. 

 The largest wires used in establishing the above formulae were 14 millims. It is 

 therefore not surprising if, after extrapolating for a wire over three times this 

 diameter, the calculated and experimental values differ considerably. This diver- 

 gence is also in some degree accounted for by the fact that the enclosures used 

 in the two cases were of an entirely different shape and size. 



In a paper recently published, Mr. C. F. BRUSH gives a number of observations 

 " on the transmission of heat by gases," at temperatures of from to 15 C. 

 Unfortunately it is impossible to reduce these to absolute units. The relative 

 value of the emissivity in air and in hydrogen is much the same as that which may 

 be deduced at some hundred degrees higher from fig. 2. My measurements show 

 that the emissivity in carbon dioxide is greater than in air at high temperatures, 

 converging to the same value at 150 C. At about 10 C., according to Mr. C. F 

 BRUSH, the relative value of the emissivity in these two gases is reversed. 



Many formula have been given, principally with regard to radiation in vacuo ; 

 but I am aware of none that will apply at atmospheric pressure and high tem- 

 peratures. 



Until more is known as to the physical constants of gases at temperatures 

 ranging from the melting point of silver to the melting point of platinum, it is 

 doubtful whether any general law can be obtained. 



* ' Wied. Ann.,' vol. 26, p. 287, 1885. 



t ' Phil. Trans.,' A, vol. 183, p. 371, 1892. 



