RESIDUO-CAPILLARY ATTRACTION. 223 



Water impregnated with sulphuretted hydrogen was found not only 

 to stop the process, but to destroy the energy of the membrane for 

 subsequent experiments with pure water and pure saccharine solutions. 

 No doubt the sulphuretted hydrogen had decomposed the surface of 

 the capillary pore, leaving a coating of putrid matter, which was not 

 possessed of such capillary properties as to supply the place of the 

 material of the membrane. That this is the true explanation is shown 

 by the fact, that when the membrane was for a long time steeped in 

 water and well washed, its energy was restored : in fact, the putrid 

 matter being washed away, the membrane presented an unvitiated 

 surface to the fluids. Heat and electricity may be classed amongst 

 these chemical agents, as they operate their effect precisely in the same 

 way, namely, by changing the chemical and consequently the capillary 

 affinities. 



20. If we wish to compute the height to which the fluid will 

 rise in the endosmometer, let ^ be the height of the supported column 

 above the surface of the membrane, and z the height of the lower fluid 

 above the same, w the transverse section of the tube; the difference 

 of the pressures of the cylindrical columns w^ and wg, having the 

 common section w, is gpco^—grioz: this must be counterbalanced by the 

 sustaining force cA(p — r) + cS{p^ — r"), which denotes a pressure on the 

 same scale ; 



" o ff \ p J oi ff \ p / p 



If a column of mercury be hydrostatically substituted for the 

 ascending fluid, as in the experiments of Dutrochet, calling Z the 

 altitude of the mercury, and R its density, we must have 



„ c A (p-r\ cB (p^-r^\ r 



^ = -.- g • K-w) ■" -.^ ■K-R-) "■ R^' 



this of course being subject to a correction when the cistern of the 

 mercury is not on a level with the membrane. 



F F2 



