Radiation of Heat by Gaseous Matter. 427 



them into the experimental tube; the absorption was fifteen 

 times that of dried air. A roll of bibulous paper, taken from 

 one of the drawers of the laboratory, and to all appearance per- 

 fectly dry, was enclosed in a glass tube, and dry air carried be- 

 tween its leaves. The experiment was made five times in suc- 

 cession with the same paper, and the following absorptions were 

 observed : — 



Absorption. 



No. 1 



• • 



. 72 



No. 2 



• • 



. 62 



No. 3 



» • • 



. 62 



No. 4 , 



• • 



. 47 



No. 5 . 



• • 



. 47 



In fact, the action of aqueous vapour is exactly such as might 

 be expected from the vapour of a liquid which Melloni found to 

 be the most powerful absorber of radiant heat of all he had 

 examined. 



Every morning, on commencing my experiments, I had an 

 interesting example of the power of glass to gather a film of 

 aqueous vapour on its surface. Suppose the tube mounted, and 

 the air of the laboratory removed, as far as the air-pump was 

 capable of removing it. On allowing dry air to enter for the 

 first time, the needle would move from 0° to 50°. On pumping 

 out it would return to 0°, and on letting in dry air a second 

 time it would swing almost to 40°. Repeated exhaustions would 

 cause this action to sink almost to nothing. These results were 

 entirely due to the vapour collected during the night in an invi- 

 sible film on the inner surface of the tube, and which was removed 

 by the air on entering, and diffused through the tube. If the 

 dry air entered at the end of the tube nearest to the source of 

 heat, on the first and second admissions, and sometimes even on 

 a third, the vapour -carried from the warm end to the cold end 

 of the tube was precipitated as a mist upon the latter, for a 

 distance sometimes of nearly a foot. The mist always disap- 

 peared on pumping out. It is needless to remark that facts of 

 this character, of which I could cite many, were not calculated 

 to promote incautiousness or rashness on my part. I saw very 

 clearly how easy it was to fall into the gravest errors, and I took 

 due precautions to prevent myself from doing so. 



Knowing that a solution of salt was almost as opake to ra- 

 diant heat as water itself, I was careful to examine whether the 

 effects which I had observed with aqueous vapour might not be 

 due to the precipitation of the vapour on the surfaces of the 

 plates of salt used to stop my tube. The substance is well 

 known to be very hygroscopic; and during the last three years 



2 F2 



