Laws of Evaporation and Absorption. 339 



the tension of the vapour. But it was found that the maximum 

 weight of moisture absorbed was the same, or very nearly the 

 same, for all temperatures or tensions of vapour. Thus a piece 

 of clean woollen cloth, weighing 398 grains when perfectly dry, 

 absorbed 92*5 grains of moisture from an atmosphere saturated 

 with vapour at 15° C. ; whereas at 5° C. the weight of moisture 

 absorbed was 91 '4 grains. Moreover, it was found that, when 

 this cloth was exposed to the air, the weight of moisture absorbed 

 (approximately) varied directly as the tension of the vapour in 

 the air at the time, and inversely as the tension of the vapour 



of saturation, as expressed in the formula w = 92—. Thus, when 



the temperature of the air was 9° C, and the tension of its 



vapour, as determined by DanielPs hygrometer, was *26, the 



weight w of the moisture absorbed by the cloth was found to 



•26 

 be 70'6 grains ; but by the formula we find w — 92 x ^r= 70'9. 



Again, when the temperature was 14°*8 C, and the tension of 

 the vapour p' was '31, the weight w was found to be 59 grains ; 



'31 

 but by the formula w = 92 x -75 



58-2. 



When p ! =p, then 



w = 92, which is the weight of the maximum absorption, as 

 before given. Hence it would appear that a piece of clean 

 woollen cloth might be used as a hygrometer, probably giving 

 nearly as reliable results as the ordinary form of the wet- and dry- 

 bulb thermometers. Thus we should have »' = — , the constant 



a 



a being the maximum amount of absorption of the piece of 

 cloth employed. 



On the Diffusion of Vapour in the Air. 



8. The diffusion of vapour from a damp surface through a 

 variable column of air, varies (approximately) in the inverse ratio 

 of the depth of the column, the temperature being constant. 



Experiment VIII. — In this case Experiment VII. was re- 

 peated at constant temperature, 14° C, but with a variable 

 depth of the acid from the surface of the plate D. 



Depth of the acid 



from the damp 



surface, in inches, 



D. 



Corresponding weight , Value of w by formula 

 of vapour absorbed, 21 

 in grains, «-'= — • 

 w. 



1 

 2 

 3 



H 



210 210 



10 4 10-5 



6-6 70 



5-8 GO 



