Laws of Evaporation and Absorption. 341 



corresponding time being 7*5 minutes ; the times corresponding 

 to o, 9 and 0°*6 below the temperature of saturation were found 

 to be 73 and 92 minutes respectively. 



Here it will be observed that the times for the same di- 

 stance from the point of saturation nearly coincide with each 

 other. It will be further observed that the maximum depres- 

 sions are pretty nearly as the tensions of the vapour at the re- 

 spective temperatures. 



The rationale of this result would appear to be this : whilst 

 the rate of diffusion increases as the tension of the vapour at 

 the particular temperature, at the same time the amount of 

 vapour to be diffused also increases according to the same law. 

 Assuming that the rate of evaporation, v 3 varies as p—p ] } we 

 have 



■j^=v = a(p-p'); .:T = q\og^ i =q log j^ r 



Now if — =a constant, then T = a constant; that is, the 



P 

 times of diffusion are equal for equal degrees of saturation. 



When p' =p, T = cc ; that is, theoretically the time of complete 



saturation is infinite ; but practically the time requisite to 



saturate the air within a small limit is definite. 



10. The times in which different volumes of dry air become sa- 

 turated with watery vapour (or saturated within a given relative 

 degree) are nearly proportional to the volumes. 



Experiment X. — The foregoing experiment was performed 

 with a bottle containing three half-pints; that is, in this case 

 the volume of the air was one-fifth of the volume of the air in 

 the last experiment. 



The temperature being 13° C, the maximum depression was 

 found to be 2°*3, the time being 4 minutes, and the time cor- 

 responding to o, 6 below the temperature of saturation was 

 found to be 18 minutes, whereas for five times the volume of 

 air it was found to be 92 minutes, that is, nearly five times the 

 time. 



If water be placed at the bottom of a deep open jar, without 

 soiling the sides, the humidity of the air within the jar, after it 

 has stood for several hours, is nearly uniform, and only slightly 

 in excess of that of the external air. 



Experiment XI. — The jar used in this experiment was 18 

 inches in depth and 4 J inches in diameter. After the jar had 

 stood, with the water covering its bottom, for three hours, the 

 temperatures at the bottom, at the middle, and at the top of the 

 jar were found, by means of a thermometer with the naked 

 bulb, to be 13°6, 13°6, and 13°5 C. respectively. The wet- 



Phil. Mag. S. 4. Vol. 25. No. 169. ikfay 1863. ' 2 A 



