78 BIOLOGY. [BOOK n. 



for instance the more dense can be placed above the less dense. 

 As essentially there is a very great analogy between diffusion and 

 osmosis, it follows as a matter of course that the substances 

 which osmosis finds the most sluggish must be the colloidal 

 substances ; and that is exactly what experience confirms. In 

 osmosis, the membranous partition, usually organic, which 

 separates the liquids, is 'traversed simultaneously by a double 

 current ; and commonly the stronger current goes from the less 

 dense liquid to the more dense liquid. There are however 

 exceptions. For instance, if water and alcohol are separated by 

 a fragment of bladder, the water passes in greater quantity 

 towards the alcohol. It has been supposed that in this case 

 the direction of the current depends on the inequality of the 

 capillary attractions between the liquids and the two faces of the 

 membrane. The water moistening the membrane better than the 

 alcohol, rises by capillarity in its pores, while on the contrary if 

 we substitute for the bladder a layer of collodium, which is better 

 moistened by the alcohol than by the water, the direction of the 

 osmosis is reversed. All the membranes with which osmotic 

 experiments have been made are in .effect really and thoroughly 

 perforated (bladder, collodium, paper, parchment, and so on). 

 But this explanation does not suit all cases, and especially the 

 cases of osmosis through living liquids. In effect, so far as our 

 most powerful microscopes permit us to -assure ourselves thereof, 

 the surfaces of the animal and vegetal cells and fibres are 

 absolutely homogeneous. We find no trace of pores. No doubt 

 we are compelled to admit molecular and atomic intervals across 

 which the passage of the solutions must be effected ; but in this 

 case the osmosis is accompanied by a chemical action exercised 

 on the dialysing membrane. The liquids, gases, or vapours, which 

 traverse the cellular or fibrillary walls unite themselves, as they 

 pass along, molecule by molecule, to the chemical elements 

 constituting this wall; then, as on the other side of the 

 membrane they find themselves in contact with a new fluid, they 

 forsake forthwith the elements of the wall to combine with those 



