318 Mitchell on the Penetrativeness of Fluids. 



tion of ascent ensued. At length, so little was received, as to barely 

 compensate for the effect of gravitation. Finally, the diminished 

 power of reception being below the effect of gravitation, the liquid 

 descended again, and the two columns reached a common level. 

 Seeing these causes of change, we can estimate the rate solely by 

 observing the time taken to traverse a short space, and that imme- 

 diately at the commencement of the experiment. Unless the less 

 penetrant liquid be of much more power of reception than is actually 

 necessary, its dilution soon destroys its adequacy, and lessens the 

 apparent rate, just as, in forming solutions, we perceive a great 

 diminution of solvent power as the point of saturation is approached. 

 In addition, when both liquids are traversing the membrane at the 

 same time, there is a progressive approach to a common state, fa- 

 vourable to repose. M. Dutrochet, therefore, by observing the effect 

 of solutions of different strength, in a considerable length of time, 

 (an hour and a half,) obtained results, not the act of the membrane, 

 but of the solution not the maximum effect of the tissue, but the 

 constantly diminishing action on water of a gradually diluted solu- 

 tion. His results might therefore have been anticipated by calcu- 

 lation ; for, as water dissolves less and less, in a given time, of any 

 soluble substance, so a soluble substance acts on water presented 

 to it in a steadily declining ratio. When the demand for water is 

 above the powers of supply through the membrane, the rate will be 

 regulated solely by the water and membrane, and is the same for 

 a great variety of substances. When the demand becomes less than 

 the supply, the case is one of simple solution, with which the mem- 

 brane may be supposed to have no connexion. It is then acting 

 the part of a still surface of water. 



The following facts, ascertained at an early period of this investi- 

 gation, will place this principle in a strong light. An inverted sy- 

 phon, such as already described, was filled with atmospheric air, a 

 portion of which, by placing thirty-four inches of mercury in the 

 long limb, was confined in the shorter one. There being here the 

 same gas on both sides of the membrane, the current set in the di- 

 rection given by impulsion, and the long column fell 



^ths of an inch in 2 hours and 30 minutes, or 50 min. per ^th. 



-| ths more in 2 hours and 39 minutes, or 53 per ^th. 



Jths more in 2 hours and 26 minutes, or 48J- rds per -^th. 



ith in 1 hour and 1 minute, or 61 per -|th, 



1 inch in the whole in 8 hours and 36 minutes. 



