190^] Osmotic Phenomena. 485 



WEEKLY EVENING MEETING, 

 Friday, February 26, 1009. 



Sir William Crookes, D.Sc. F.R.S., Honorary Secretary and 

 Vice-President, in the Chair. 



Professor H. L. Callexdar, M.A. LL.D. F.R.S. 



Osmotic Phenomena and their Modern Physical Interpretation. 



Osmotic pressure is a phenomenon of such importance in the theory 

 of solutions, and in the interpretation of all vital processes, and so 

 much valuable work has recently been directed to its elucidation, 

 that, although it is a somewhat thorny and diificult subject, no 

 apology is needed for any serious attempt, however inadequate, at its 

 explanation. 



One of the earliest recorded experiments on osmotic pressure is 

 that of the Abbe Nollet, who found that a bladder containing alcohol, 

 when immersed in water, absorbed water so greedily as in many cases 

 to burst the bladder. The experiment illustrates in an imperfect 

 manner the fundamental property of all animal and vegetable mem- 

 branes of allowing some substances to pass through them by osmosis 

 more easily than others. In many cases such membranes, while freely 

 permeable to water, are practically impermeable to certain substances 

 in solution, and play the part of sieves in directing and controlling 

 diffusion. It will readily be understood that results of the greatest 

 importance to biology have been obtained by studying this property 

 of semipermeahility, as it is called, but the application of natural 

 membranes to the physical study of the subject is necessarily limited 

 on account of the difficulty of obtaining sufficiently large and perfect 

 membranes capable of withstanding any considerable pressure. 



Artificial membranes of sufficient fineness to be impermeable to 

 such substances as sugar in solution, were first prepared by Traube by 

 means of precipitated pelHcles of substances hke copper-ferrocyanide. 

 The first quantitative measurements of osmotic pressures of consider- 

 able magnitude were made by Pfeffer with membranes of this kind 

 deposited in the pores of earthenware pots fitted with suitable mano- 

 meters for indicating the pressure developed. Pfeffer found that 

 when a semipermeable vessel, filled with sugar solution, was immersed 

 in water, the membrane being freely permeable to water, but not to 

 the dissolved sugar, the solution absorbed water through the mem- 

 brane by osmosis until the internal pressure reached a certain magni- 

 tude sufficient to balance the tendency to absorption. The osmotic 



