Radiant Heat by Dry and by Moist Air. 



247 



the magnet-mirror was now made with telescope and scale, and 

 in such a manner that the position was always read off on the 

 scale as soon as, by continued pumping, the magnet was brought 

 approximately to rest. The position of the magnet previous to 

 the forcing-iu of air was not noted, but instead of this the deflec- 

 tion to the opposite side was observed, consequent upon the pre- 

 viously-described interchange of the caoutchouc tubes, whereby 

 the nature of the air in each of the two experimental tubes was 

 altered. In this manner the following numbers were obtained : — 



Left tube. 



Right tube. 



Scale-reading. 



Difference. 



Moist air 



Dry air 



Moist air 



Dry air 



Moist air ... 



Dry air 



Moist air 



Dry air 



Goal-gas 



Air iii room... 



Dry air 



millims. 

 520 

 410 

 440 

 330 

 490 

 370 

 440 

 340 

 beyond 1000 

 beyond 



millims. 

 110 



110 



120 



100 



Moist air 



Dry air 



Moist air 



Dry air 



Moist air 



Dry air 





Air in room ... 





Decimetres rather than millimetres were noted here, since 

 the magnet never came quite to rest. A heating of the right 

 side of the thermopile produced a deflection towards the higher 

 divisions of the scale. Lastly, the temperature of the room, and 

 therefore also that of the air saturated with aqueous vapour, was 

 18° C. Assuming that the air on entering the tubes spreads 

 solely to the side where the suction by the air-pump takes place, 

 and that it there immediately leaves the tube, the interposed 

 strata of moist and of dry air in the tubes would each be 

 only 30 centims. thick; from the foregoing experiments, there- 

 fore, it would follow that by replacing a stratum, of dry air 30 

 centims., or 1 foot thick, on one side of the thermopile by 

 air saturated with aqueous vapour at 18° C, the thermal action 

 of the source of heat at 100° on the corresponding end of the 

 thermopile would be so far diminished as to cause a deflection of 

 the magnet of our galvanometer amounting to 55 millims. of the 

 scale, or, bearing in mind the stated distance of the scale from 

 the mirror, to about ^°. 



In order to be able to compare this absorption by aqueous 

 vapour with that by coal-gas, for which purpose the above-men- 

 tioned experiments with coal-gas could not serve, the position 

 of the magnet-system was so changed, by means of the auxiliary 

 magnets, that the zero of the scale nearly appeared in the tele- 

 scope when both tubes were filled with the air of the room. 



