418 Prof. H. BiifFon tJie Thermal Conductivity 



Table IX. 



T. 



T'. 



t. 



T. 



z'. 



millim8. 



100 T 



t ' 



490 



360 



30-3 



23-9 



(24-5) 

 271 





755 



809 



55 



39-0 



33-9 





745 



85-5 



570 



390 



35-0 



(29-0) 

 27-0 



(28-9) 





745 



82-6 



423 



257 



25-7 



24-6 





92 



97-7 



56-0 



320 



320 



(255) 

 30-0 





7-8 



1050 



500 



31-3 



313 



(33-6) 

 281 



(30-5) 





Tl 



97-4 



In the first three experiments the air in the glass cylinder 

 was of atmospheric density, in the last three it was greatly 

 rarefied. Of about 100 rays which passed through the rarefied 

 space and the rock-salt, only about 83 were transmitted through 

 the denser air. 



The thermal colours of the two substances are not perfectly 

 alike, but sufficiently similar to explain the phenomena ob- 

 served by Tyndall. 



Tyndall determined the absorptive capacity of dry air from 

 the very small alteration caused by filling the previously ex- 

 hausted tube of 4 feet length with dry air. After what has 

 been stated it will not be surprising that other gases, such as 

 moist air and defiant gas, possessing other thermal colours, 

 when substituted for dry air in the tube, produced greatly dif- 

 ferent effects upon the galvanometer. But such differences 

 do not prove a greater absorptive power of the respective 

 gases, as Tyndall supposed, but, in the first instance, a difference 

 only in their thermal colours. 



On comparing the small effect of dry air upon the needle 

 with the greater one produced by moist air, as resulting from 

 his mode of observation, Tyndall arrived at the remarkable 

 conclusion that the thermal absorption of the latter is from 20 

 to 40 times as great as that of the former. This conclusion 

 was called in question by Magnus, who had found in his ex- 

 periments with dry and moist air, when not using a plate of 

 rock-salt, that the difference in their absorptive powers for rays 

 of the temperature of boiling water is very small, though that 

 of moist air is somewhat the greater. Thus originated the well- 

 known controversy between the two scientific men — a contro- 

 versy which excited all the more interest, inasmuch as both 



