and Diathermancy of Air and Hydrogen. 



409 



As currents seemed unavoidable under these circumstances, 

 more especially in the mobile hydrogen, and as thus there was 

 sufficient ground to suppose that these currents were the only 

 cause of the greater thermal motion of hydrogen, it appeared 

 indispensable to maintain a constant change of the cooling- 

 water. This was done by drawing off the water by means of 

 a siphon from the upper part of the annular space, whilst at 

 the same time cold water was introduced at the bottom. By 

 this arrangement the temperature of the cooling-Avater was 

 maintained so uniform that it did not change more than 0*1 

 to 0'2 degree in the course of an observation. 



In addition to this, the soldered junction of the thermopile 

 was lowered from 23 to 45 millims. below the bottom of the 

 brass vessel ; this distance was still some millimetres less than 

 that Magnus had in his experiments. By this means, as well 

 as by the cooling-water, the possibility of the glass becoming 

 warm down to the level of the soldered junction was consi- 

 derably diminished. 



The following figures were now obtained for dry hydrogen 

 and air: — 



Table IV. 





T. 



t. 



r. 



z. 



P- 



100 r 



t ' 



Hydrogen ... 



fllO 



J46-0 

 [39-6 



28-6 



11-5 



8 



7600 



40-2 



Hydrogen ... 



( 11-0 



^55-3 

 [47-5 



36-5 



15-9 



7 



755-0 



43-6 



Air.. 



rii-o 



J52-0 

 l_45-3 



34-3 



7-5 



7 



744-5 



21-8 





Air 



[ 10-5 

 ^51-0 

 1 43-0 



32-5 



13-5 



7 



1-5 



40-0 





Air . . 



[ 9-4 

 \ 600 

 [51-3 



41-9 



16-5 



7 



1-5 



41-5 





On comparing, as exhibited by these figures, the relation of 

 the thermal motions in hydrogen and in vacuum, an almost 

 entire similarity is obsers^ed. Now the motion in vacuum is 

 almost exclusively due to radiation. The conclusion seems 

 therefore justified, that hydrogen is as diathermanous to rays, 

 emanating from a body heated by means of hot tvater, as a 

 vacuum. 



