368 On the Laws of Absorption of Liquids by Porous Substances. 



Obs. In accordance with observations before made, the value 



B 



of the velocity derived from the formula r=^= only holds true 



when T is large compared with p. 



Nearly the same results were obtained with an absorbent of 

 one-half the width. Hence it appears that the rate of diffusion 

 is not dependent upon the width of the absorbent. 



This experiment was repeated with the vessel AB exhausted 

 of air. The results were nearly the same as those above recorded. 

 Hence it would appear that the rate of diffusion is not affected by 

 the presence of air. 



Rate of evaporation. — In order to determine the rate of evapo- 

 ration from the surface of an absorbent saturated with moisture, 

 the absorbent of this experiment was taken out of the water and 

 suspended in the air from the scale of a delicate balance, the dry 

 bulb of the thermometer being 62°, and the wet bulb 58°. The 

 weight of the whole water absorbed was found to be 24'3 grs., 

 that is, 1*215 gr. per square inch of surface. The weight lost 

 during each successive interval of 30 minutes was found to be 

 very nearly 4-74 grs. At the end of 2 J hours the rate of evapo- 

 ration sensibly declined, and the last few grains of moisture were 

 retained with considerable tenacity. Hence it appears that the 

 rate of evaporation, from the surface of an absorbent saturated 

 with moisture, is for the most part uniform. 



An absorbent thus placed in contact with water (as will be 

 hereafter more fully shown) becomes saturated with moisture ; 

 and when it is exposed to the action of the atmosphere, it becomes 

 equally dry at all parts in the same time. But if the whole 

 surface of the absorbent be plunged into the water and then 

 suspended in the air, the lower portion of the absorbent retains 

 a sensible film of water by adhesion to the surface ; this portion 

 of the absorbent maybe said to be supersaturated with moisture, 

 and it will be found that the higher portion of the absorbent will 

 become dry much sooner than the lower, and also that the rate 

 of evaporation will not go on uniformly. 



The absorbent of this experiment, after being dried, was 

 suspended in contact with water as usual, and exposed to the 

 air. For the first 4 inches of the ascent of the liquid through 

 the absorbent, the law of ascent closely corresponded with that 

 of the foregoing experiment ; but after this the rate of ascent 

 rapidly decreased, until the liquid had risen to the height of 

 about 8 inches, where it appeared to become stationary. At 

 this time the dry bulb of the thermometer indicated 65°- 5, and 

 the wet bulb 62°, giving a difference of 3 0, 5. When the differ- 

 ence between the indications of the two bulbs had become 5°, 

 the liquid stood at the height of 9 inches ; and when this differ- 



