OSMOTIC PHENOMENA 33 



this free side were serving as an osmotic membrane or not, 

 for pressure produced anywhere in the bag must be trans- 

 mitted equally and undiminished to all parts of the surface, 

 in accordance with Pascal's principle of transmission of 

 pressure in fluids. This transmission would be accomplished 

 by the fluid as a whole, solvent and solute acting together. 

 If, however, the portion where the bulging is supposed to 

 take place be permeable to the solvent, the immediate pres- 

 sure which affects the membrane must be due to the solute. 

 Thus it comes to the same end if we consider the pressure 

 as transmitted by the solute, acting like a gas ; for increased 

 energy of solvent particles will be transmitted to the solute 

 particles with which they come in contact. 



A very common mode of demonstrating the existence of 

 osmotic pressure is the following : A piece of animal bladder 

 or parchment paper is tied tightly over the expanded end of 

 a thistle tube, and the bulb is filled with molasses, or a strong 

 aqueous sugar or salt solution. Then the tube is fastened 

 upright, the bulb being immersed in water so that the liquids 

 within and without have a common level. After a time it is 

 observed that the solution has risen in the tube, often to the 

 height of a meter or more if the tube is sufficiently long. 

 The diffusion tension of the solute particles within the bulb 

 is of course operative in every direction, but osmotic pressure 

 is developed and made apparent only within the membrane. 

 This pressure is transmitted through the liquid to all parts 

 of the surface of the solution. But the only part of this 

 surface which is free to move, after the limit of extensibility 

 of the membrane is reached, is the free surface in the stem 

 of the tube. We have seen that the free surface of a liquid 

 is bounded by a peculiar layer or film. Upon this film the 

 transmitted osmotic pressure is effective, just as though the 

 film were a piston closely fitting within the bore of the tube. 

 In this case, since the surface layer is nearly impermeable to 



