Radiant Heat to Aqueous Vapour, 85 



The tube A B being* filled by the common air of the laboratory, 

 and the needle of the galvanometer pointing steadily to zero, dry 

 air was forced gently from the india-rubber bag through the 

 cock c ; the pump was gently worked at the same time, the dry 

 air being thus gradually drawn towards d. On the entrance of the 

 dry air, the needle commenced to move in a direction which 

 showed that a greater quantity of heat was now passing through 

 the tube than before. The dry air proved more transparent than 

 the common air, and the final deflection thus obtained was 41 

 degrees. Here the needle stopped, and beyond this point it 

 could not be moved by the further entrance of dry air. 



Shutting off the india-rubber bag and stopping the action of 

 the pump, the apparatus was abandoned to itself; the needle 

 returned with great slowness to zero, thus indicating a corre- 

 spondingly slow diffusion of the aqueous moisture through the 

 dry air within the tube. By working the pump the descent of 

 the needle was hastened, and it finally came to rest at zero. 



Dry air was again admitted ; the needle moved as before, and 

 reached a final limit of 41 degrees ; common air was again sub- 

 stituted, and the needle descended to zero. 



The tube being filled with the common air of the laboratory, 

 which was not quite saturated, and the needle pointing to zero, 

 air from the india-rubber bag was now forced through two 

 U-tubes filled with fragments of glass wetted with distilled water. 

 The common air was thus displaced by air more fully charged 

 with vapour. The needle moved in a direction which indicated 

 augmented absorption ; the deflection obtained in this way was 

 15 degrees. 



I have repeated these experiments hundreds of times, and on 

 days widely distant from each other. I have also subjected them 

 to the criticism of various eminent men, and altered the conditions 

 in accordance with their suggestions. The result has been inva- 

 riable. The entrance of each kind of air is always accompanied 

 by its characteristic action. The needle is under the most com- 

 plete control, its motions are steady and uniform. In short, no 

 experiments hitherto made with solids and liquids are more free 

 from caprice, or more certain in their execution, than are the 

 foregoing experiments with dry and humid air. 



The quantity of heat absorbed in the above experiments, ex- 

 pressed in hundredths of the total radiation, was found by 

 screening off one of the sources of heat, and determining the 

 full deflection produced by the other and equal source. 



By a careful calibration, repeatedly verified, this deflection was 

 proved to correspond to 1200 units of heat, — the unit being, as 

 before, the quantity of heat necessary to move the needle of the 

 galvanometer from 0° to 1°. According to the same standard, 



D2 



