106. Prof. Magnus on the Propagation of Heat in Gases.. 



temperature (not 2 per cent, of its volume at 16° C.) could exercise 

 no influence on the radiation, it appeared desirable to determine 

 experimentally that this supposition was correct. With this view 

 I made comparative determinations of the radiation through 

 perfectly dry air and through air entirely saturated with moisture. 

 The air was passed through several chloride of calcium tubes, 

 and afterwards, by means of a respirator, was drawn through the 

 unblackened tube in such quantity that all the air previously in 

 the tube might be considered as displaced. After the radiation 

 was determined, the air was exhausted by means of the air-pump, 

 and fresh air admitted, which before entering had passed through 

 water slowly and in small bubbles. This air was then exhausted, 

 and a fresh quantity admitted under the same circumstances. 

 After moist air had been thrice successively admitted, it could 

 be assumed that the whole of the air contained in the tube at 

 the temperatnre 16° C, and pressure 7646 millims., was satu- 

 rated with aqueous vapour. 



The capacity of dry air and moist air to transmit heat-rays of 

 100° C. was investigated in exactly the same way by means of 

 the apparatus described in page 87. In this way the following 

 results were obtained : — 



Dry air. 



Air saturated with aqueous vapour. 



Position of 

 equilibrium! „ r 



of the Mean - 



needle. 



Observed 

 deflection. 



Difference. 



Position of 



equilibrium 



of the 



needle. 



Observed 

 Mean " deflection. 



Difference. 



With the Gas-lamp. 



6-o 



-0-5 

 00 

 00 

 00 



-0-25 



-0-25 



00 



00 



o 



11-5 

 11-5 

 120 

 120 



o 



1175 

 1175 

 120 



120 



-1-5 



-1-25 



-1-25 



-1-25 



-1-25 



o 



-1-35 

 -1-25 

 -1-25 

 -1-25 



1075 

 105 

 1075 

 105 



o 



121 

 1175 

 120 

 1175 



Mean 12-0 



Mean 12-0 



With the source of heat at 100° C. 



00 

 00 

 00 

 00 





12-5 

 1275 

 12-5 

 12-5 





+05 

 +0-5 

 +05 

 +0-5 

 + 10 



+05 



+0-5 

 +0-5 

 +075 



13 25 

 13-25 

 13 

 13-5 



1275 

 1275 

 125 

 1275 



Mean 12-5 



Mean 126 



These experiments show that the water present in the atmo- 

 sphere at 16° C. exercises no perceptible influence on the radia- 

 tion. That such an influence should be felt as soon as part of 

 the vapour separates as fog, appears very probable. 



