Conditions of Molecular Action. 367 



of those particles of fluid A which previously filled the pores of 

 the septum (those particles of fluid A perhaps alone remaining 

 unmixed which were in close contact with and lining the sides 

 of the pores) . 



We may now easily surmise the mode in which the opposite 

 currents are produced. The particles of each fluid at each end 

 of the pores, from their miscibility, will be continually taken up 

 by the opposite fluids (those of fluid A by fluid B, and vice 

 versa), and thus a flow of each fluid one into the other will be 

 started ; and since upon one fluid (A) a double influence is ex- 

 erted, by both fluid B and the septum, its diffusion will proceed 

 at a greater rate. The currents will continue to flow until the 

 affinities of each are satisfied or are in equilibrium. 



It seems possible that the size of the pores may to a certain 

 extent determine the direction of the current, since upon this 

 circumstance depends the quantity of fluid in contact with the 

 septum ; and this, if it were true, would assist the solution of 

 many vital problems. 



In this explanation of osmosis it will be seen that the chief 

 part in the action has been attributed to molecular affinities 

 similar to those which produce solution and mixation — with this 

 difference, that in the septum between the fluids we have certain 

 mechanical conditions and a rigid insoluble material concerned, 

 instead of a fluid or a soluble solid*. 



Dialysis differs from osmose in the extent to which the current 

 from one fluid (the crystalloid) prevails over that of the other 

 (the colloid) ; it is in fact little more than the diffusion of the 

 crystalloid through the colloid. 



A colloid possesses all the qualities most favourable to mole- 

 cular (as distinguished from chemical) action. Its particles have 



* The account given of endosmosis by M. Lhermite {Annates de Chimie 

 et de Physique, vol. xliii. p. 420) is, in the main, similar to that just stated, 

 except that he scarcely asciibes sufficient importance to the molecular in- 

 fluence exerted by the two fluids one upon the other. Thus he thinks 

 that the passage of the second fluid (B), which is less influenced by the 

 septum than fluid A, is purely passive and due to pressure, and that if the 

 pores of the septum were ever to become entirely lined by fluid A the 

 osmotic action would cease. His experiments, however, give striking evi- 

 dence of the general correctness of the views which I have endeavoured to 

 elucidate. Thus he shows that a kind of osmosis takes place when two 

 miscible fluids, as water and alcohol, are separated by a third fluid (such as 

 essence of turpentine), or ether and chloroform separated by water. And 

 the influence of the intervening septum is well illustrated by an experi- 

 ment made with alcohol and water separated by a diaphragm of porous 

 earthenware. In this case the chief current is from the water into the 

 spirit ; but if the cylinder, after careful drying, be steeped for three or four 

 days in castor oil and then again used, the alcohol will pass over in greatest 

 amount into the water* 



