292 Dandeno : Osmotic theories 



One other illustration of a conclusion arrived at from the gas- 

 pressure theory maybe given. The statement has been made that 

 if a solution strong osmotically were placed in a very thin glass flask 

 (such as that used by physicists to demonstrate atmospheric pres- 

 sure by exhausting the air and causing the flask to collapse), the 

 flask would immediately explode because of the *' bombardment of 

 the molecules in solution.*' The experiment is the best proof of the 

 fallacy of this notion. There is no osmotic pressure whatever in 

 such a case and no explosion either. All these theories fall to the 

 ground when examined closely. Now, it is not the intention here 

 to show the insufficiency of these theories merely. The chief ob- 

 ject in writing this discussion is to present a clear and reasonable 

 theory, to explain osmotic action and particularly osmotic pressure ; 

 a theory is valuable only so long as it explains. 



The theory proposed here postulates two things : (l) That a 

 membrane can be less permeable to one liquid than to another. 

 (2) That the molecules of all substances which are soluble in si 

 certain solvent have an attraction for the molecules of the solvent 

 and the molecules of the solvent for the substance in solution. In 

 regard to the former of these two postulates, it may be said it is 

 entirely comprehensible that a membrane may permit one body to 

 pass through a pore more readily than another, because of the 

 relative size or shape of pore and molecule, and because of friction 

 offered by the membrane to the passage of the molecule. The 

 second postulate above mentioned refers to a condition so common 

 in naturd that it scarcely needs a word of comment All mole- 

 cules are probably endowed by nature with this attractive force, 

 but it must not be forgotten that this force operates only at very 

 minute distances. To be more explicit, let us take an example. 

 Choosing the common osmometer having a sugar solution within 

 and water without the pig's bladder membrane ; suppose the sugar 

 molecule to be too large to pass through the pores of the bladder, 

 and that these pores are large enough to permit the water mole- 

 cules to pass with little or no opposition, the molecules of sugar 

 attract the molecules of water and both tend to move toward each 

 other. The sugar molecules are prevented from moving through 

 the membrane, but the water is not, hence there is an inflozv of 



water, due to attraction. 



the/^ 



