36 Prof. Tyndall on the Relation of 



a deflection of 41° corresponds to an absorption of 50 units. 

 From these data we immediately calculate the number of rays 

 per hundred absorbed by the aqueous vapour, 



1200:100 = 50:4-2. 

 An absorptioD of 4*2 per cent, was therefore effected by the 

 atmospheric vapour which occupied the tube between the points 

 c and d. Air perfectly saturated on the day in question gave an 

 absorption of 5-J- per cent. 



These results were obtained in the month of September, and 

 on the 27th of October I determined the absorption of aqueous 

 vapour with the above tube when stopped with plates of rock- 

 salt. Three successive experiments gave the deflections pro- 

 duced by the aqueous vapour as 46°-6, 46°-4, 46°'8. Of this 

 concurrent character are all the experiments on the aqueous 

 vapour of the air. The absorption corresponding to the mean 

 deflection here is 66. The total radiation through the exhausted 

 tube was on this day 1085 ; hence we have 

 1085:100 = 66:6-1; 

 that is to say, the absorption of the aqueous vapour of the air 

 contained in a tube 4 feet long, was on this day 6 per cent, of 

 the total radiation. 



The tube with which these experiments were made was of 

 brass, polished within ; and it was suggested to me that the 

 vapour of the moist air might have precipitated itself on the 

 interior surface of the tube, thus diminishing its reflective power, 

 and producing an effect apparently the same as absorption. In 

 reply to this objection, I would remark that the air on many of 

 the days on which my experiments were made was at least 25 

 per cent, under its point of saturation. It can hardly be sup- 

 posed that air in this condition would deposit its vapour upon a 

 polished metallic surface, against which, moreover, the rays from 

 our source of heat were impinging. More than this, the absorp- 

 tion was exerted even when only a small fraction of an atmo- 

 spere was made use of, and found to be proportional to the quan- 

 tity of atmospheric vapour present in the tube. The following 

 Table shows the absorptions of humid air at tensions varying 

 from 5 to 30 inches : — 



Tensions 

 in inches. 

 5 



10 



15 



20 



25 



30 



Abs< 



Drption. 





A 



Observed. 



Calculated. 



16 



16 



32 



32 



49 



48 



64 



64 



82 



80 



98 



96 



