Dr. A. Fick on Liquid Diffusion. 87 



as it were^ a diffusion-current of infinite strength, and therefore 

 again immediately draw the requisite quantity of water. Now 

 this quantity of water, which, cateris paribus, during the unit of 

 time, can diffuse itself into the saturated solution without sensibly 

 altering the concentration at that point, depends ixpon the easy 

 mobility of the particles of the solution. The space in which 

 the compensation takes place must therefore also be, cceteris 

 paribus, so much shortei", and consequently the stream of water 

 towards the saturated solution stronger, the more easily mobile 

 the particles of the solution are. I was unable to verify this 

 theoretical conclusion by direct experiment. I expected that the 

 excess of water passing through the membrane, above the quan- 

 tity of salt passing in the contrary direction, must be found 

 much smaller, when the mobility of the upper solution was im- 

 peded by the addition of chalk magma. No such diminution 

 of this excess, however, could be remarked in the experiment. 



A second conclusion from the hypothesis just developed can 

 be thus expressed : the excess of the diffusing water, above the 

 salt passing in the opposite direction, is smaller, the wider the 

 pores of the separating partition. Or if, according to FoUy^s* 

 method, we define, as the endosmotic equivalent, the quotient 

 obtained by dividing the amount of water diffused by the quan- 

 tity of salt contemporaneously passed, the endosmotic equivalent 

 is smaller the larger the pores of the partition. This conclusion 

 is confirmed by experiment. Two membranes, which differ only in 

 the diameter of their pores, cannot, it is true, be obtained ; never- 

 theless we are safely warranted in assuming, that a transparent 

 film of collodion possesses pores so very much narrower, than an 

 animal membrane formed cf interlacing fibres, that all other 

 differences between the two tissues influencing diffusion, disap- 

 pear in this difference of the size of pores ; in fact, an animal 

 membrane was found to possess a very much smaller endosmotic 

 equivalent than a collodion film ; for whdst, by the use of pig's 

 bladder, only from 4 to 6 times as much water as common salt 

 passed through, there dift'used through a collodion film, during 

 the time that an imponderable trace of chloride of sodium passed, 

 a considerable quantity of water, probably many thousand times 

 greater than that of salt. 



Let us now suppose, that, instead of pure water, a solution of 

 a certain concentration c of the same salt which is present on the 

 upper side of the membrane in concentrated solution, be placed 

 on the under side of that mcmbraiic, then the following consi- 

 derations ])rescnt themselves. All the elementary strata, from the 

 wall of the pore, to an imaginary cylindrical film of the radius r, 

 thai J'(p—r) = c, can only be filled from the top to the bottom 

 * Poggendorff's Aiinalen, vol. Ixxvjii.p. 361. 



