210 On Variation of Thermal Conductivity of Metals. 



14. If we apply the same process to the Table expressing 

 the results of the latter experimenters in a vacuous enclosure 

 at Centigrade zero, we shall find that the number + 30 has to 



vt 

 be used instead of —20; so that — +30* is very tolerably 



z 

 constant, and equal to 18650 on the average, as is shown in 

 the following abridged Table of Dulong's results. Consider- 

 ing that the range of temperature extends as high as 240°, the 

 agreement is pretty good, 



9, 



9. 



(274+0)0 



e 



-+S09. 

 9 



10-69 

 7-40 

 4-89 

 3-02 . 

 1-74 



240 

 200 

 160 

 120 



80 



11530 

 12810 

 14200 

 15650 

 16270 



18730 

 18800 

 19000 

 18050 

 18670 



Hence we may write 



or, approximately, 





(8") 



°=T&m( m+0 i 1+ M 



which is the form of equation (7), 



It may be hereafter convenient to know that an expression 

 like (8') and (8") is capable of representing the law of cooling 

 in a vacuum with great accuracy, viz, 



■0=C0. 



A + 

 B-0 



(8) 



but for our present purpose I think the equation (7) will be 

 the most convenient. 



15. The agreement of equation (7), as it stands, with Du- 

 long and Petit's results it is scarcely necessary to show by a 

 Table, since the equation has been deduced by known approxi- 

 mation from their own statement which completely expressed 

 them, and the value of the terms neglected for an excess of 

 temperature so high as 240° is perfectly evident. Nevertheless 

 I have made the calculation, and the values of the " constant" 



(267 + 0+ ^ 'TrvrvJ-j or ?T> corresponding to the successive 

 excesses of temperature 240°, 200°, 160°, 120°, and 80°, are 



