RATIO OF SPECIFIC HEATS. 15 



bolometer strip seems to luive ueen effectually cxchuled in tlio foi-m of experiment 

 stated.^ 



We may also use the Fourier ecpiation to find in what degree the platinum 

 strip coincides with or follows the temperature of the yas. For simplicity we will 

 assume that the gas temperature sinks from its original value (•&(,), at a constant 

 rate in the lapse of time. In ()tlK'r woi'ds, jmt 



S. = ^„-^/ (7) 



If now we neglect the heat tlux fi'oni the ends of the bolometer strip toward the 

 middle, 



3^ = 0, 

 and S =r 5g foi t =: 0. The integral of the general equation (8) thus becomes 



B-B^=^{\-rrrj (8) 



which for / =: cc takes the simplei' form 



S -Sj= -*- (9) 



The difference (S* — ^,) between the temperature (3) of the bolometer strip and 

 the temperatui-e (s,) of the air has therefore a maximum value of b/f'\ For the 

 platinum measuring strip (silver removed), the above constants show f — 15. 

 Hence the bolometei- will coincide in temperature with the air after about 1/1 5th 

 second. However, since Ji has been taken very decidedly too small, the real case 

 is correspondingly more favorable. With this deduction our observations agree ; 

 for the bolometer reached the stationary state immediately after the noise due to 

 outrush of gas on expansion had subsided. 



The last of our sources of error, viz., internal radiation, remains to be dis- 

 cussed. By this agency the bolometer permanently receives heat fi'oni the environ- 

 ment, since the walls of the receiver B retain their initial temperatui'e Ty But 

 this heat, which is proportional to T^ — 7 ,, may be computed only if the values of 

 the emission and absorption coefficients of the reciprocating bcxlies were known. 

 In the absence of satisfactory data for these quantities we made an endeavor to de- 

 termine the effect of i-adiation experimentally. 



AVith this end in view, we covered the silver-free part of the bolometer strip, 

 galvauically, with platinum black,- and then repeated the expansion experiments 



- If with the same constants and in the same manner the fall of temperature be computed 

 for our original device of a bolometric spiral of silver wire, .004 cm. in diameter, the results are 

 such as fully to account for the difference between our earlier values and the present. 



' According to the recipe given by Lummer and Kurlbaum, c/. Ver/i. der Physik. Gesell., 

 Berlin, June 14, 1895. 



