C. C. Hutchins — Radiation of Atmospheric Air. 363 



which may be taken as fairly representing the radiation of air 

 in an ordinary room under average conditions, the sheet of air 

 being l cm thick. 



Effect of thickness of radiating column. 



It was noted at the outset that the galvanometer deflection 

 was not proportional to the thickness of the heated column of 

 air. 



A flat sheet iron pipe was made 100 cm long, 10 cm wide and 

 2.5cm thick. This pipe was substituted for the one previously 

 used. The air exit was from a pair of jaws, one fixed, one 

 movable, so that the thickness of the air column at its escape 

 could be regulated at pleasure. Measures with this apparatus 

 were made on eight days. The results were recorded as the 

 amount of galvanometer deflection per degree of t — t'. 



With openings less than l cm , no difference in the amount 

 of radiation can be detected. With larger openings a small 

 increase is observed. The results of six determinations dis- 

 tributed over four days are here given. 



Opening, 



0-5™ 



•1 cm 



2 .cm 



gem 



Deflection per degree, 



0-193 



0-195 



0-245 



0-259 



We observe that radiation is very largely from the surface 

 of contact between the hot and cold air, which seems to indi- 

 cate that a heated gas absorbs all or nearly all those rays that 

 it itself emits and that radiation takes place only, when there 

 is a fall of temperature within the limits of molecular action. 



Wave length of rays. 



It would probably be hopeless to undertake a prismatic analy- 

 sis of the rays from heated air. We can however get a sug- 

 gestion as to the length of these waves from their behavior 

 towards an absorbing medium. A very pure plate of quartz 

 0'5 cm thick was placed over the opening to the thermopile. 

 With the air at 100° no deflection could be obtained, and even 

 when the temperature was raised to 200° it was decided after 

 many trials, that there was no measurable effect. Without 

 the quartz plate the deflection was 151. The quartz plate 

 allows 93 per cent of the sun's rays to pass it, 31 per cent of 

 rays from red hot platinum and 3*0 per cent of rays from lamp- 

 black surface at 100', being in a manner progressively opaque 

 to waves of increasing length. The waves from heated air 

 must therefore be of great length since they seem incapable 

 of penetrating the quartz in any considerable degree. 



