GOLD IN SCIENCE AND IN INDUSTRY. 



231 



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effects are in proportion to the number of these minute units in a 

 given volume. 



In applying the gas theory to the ph3'sical explanation of osmotic 

 pressure it has been the custom to regard this pressure as directly due 

 to the bombardment of the semipermeable membrane by the solute 

 molecules. But this conception completely ignores the fact that the 

 pressure developed is a hydrostatic, not a gaseous pressure, and that 

 the hydrostatic pressure results directly from the penetration, of the 

 solvent nioleeides from the other side of the partition. 



It appears to me more natural to abandon the gas analogy alto- 

 gether, to regard the molecules as in the solid and liquid condition 

 proper to their temperature, and to apportion to them their respective 

 parts in the active changes according to their obvious endowment of 

 energy. 



Applying this view to the case of a solution and a solvent sej^arated 

 by a semipermeable membrane, it 

 is seen that the pressure rises on 

 the solution side, because the pure 

 solvent molecules on the other side 

 have some advantage for the dis- 

 play of their energy over the simi- 

 lar molecules in the solution. This 

 effect in its most genercd form may 

 he attributed to the dilution of the 

 solvent hy the solute molecules. In 

 cases where the osmotic pressure 

 appears to obey Boyle's law the 

 effect is exactly measured by the 

 number of solute molecules ])er 

 unit volume. But the facts of this 

 position are in no way changed if 



the effect is taken to be due to the activity of an equal number of sol- 

 vent molecules, for we then see that each solute molecule by canceling 

 the activity of one solvent molecule on the solution side permits a 

 solvent molecule from the other side to enter the solution. 



"What the exact mechanism of this cancellation is there is at present 

 no evidence to show, and the caution originally given by Lord Kelvin 

 with reference to the undue forcing of the gas analogy must also be 

 applied to the suggestion now put forward. But as a means of mak- 

 ing the suggestion a little more clear I give here a simple diagram on 

 which A represents a single perforation in a semipermealjle mem- 

 brane, P, on both sides of Avhich there is only pure solvent. For the 

 sake of clearness the molecules are shown only as a single vow. Nor- 

 mally there will be no passage of solvent molecules from side to side, 

 for the average kinetic energy of the molecules on both sides is equal. 



