THE ENERGY OF MOLECULES IN SOLUTION 



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FIG. 20. 



this process of diffusion will cease only when the concentration has become 

 the same in all parts of the solution. Supposing however the two fluids are 

 separated by a piston, p (Fig. 20), which is ' semi-permeable/ i.e. allows free 

 passage to water, but not to the dissolved sugar, the molecules of sugar will 

 now exert a pressure on the piston similar to that exerted on the walls of the 

 containing vessel, and will tend to drive it upwards. The force which it is 

 necessary to apply to the piston to prevent its upward movement will be the 

 measure of the osmotic pressure of the sugar in the solution. If the piston 

 be pressed down with a greater force, the sugar molecules alone are pressed 

 together, since water can pass freely through the surface of the piston, and 

 the sugar solution is therefore rendered more concentrated. Since force 

 must be applied to the piston in order to press it down, work 

 is done in the process, so that the concentration of any solution 

 volves the performance of an amount of work determined by 

 e initial and final osmotic pressures of the solution. If, on 

 e other hand, a weight be applied to the piston which is 

 less than the osmotic pressure exerted by the sugar solution, 

 e piston with its weight will be moved upwards, and the 

 lution will undergo dilution until its osmotic pressure exactly 

 balances the weight on the piston. We see that the osmotic 

 pressure of a solution represents a certain amount of potential 

 energy, which can be utilised in an osmotic machine, such as 

 at represented in the diagram, for the performance of work. 



THE MEASUREMENT OF OSMOTIC PRESSURE. By a method 

 iffering but little from the one just sketched out, Pfeffer succeeded directly 

 in measuring the osmotic pressure of certain solutions. For this purpose 

 Pfeffer took advantage of the fact, discovered by Traube, that various pre- 

 cipitates, if deposited in the form of membranes, were impermeable to the 

 substances producing them as well as to some .other dissolved substances, 

 though allowing a free passage of water. Thus, if a drop of a concentrated 

 solution of potassium ferrocyanide suspended to a glass rod be introduced 

 carefully into a more dilute solution of copper sulphate, it will be observed 

 that at the junction of the drop and the surrounding fluid there is a brown 

 membranous precipitate of copper ferrocyanide. In consequence of the 

 greater concentration of the fluid in the drop, a constant passage of water 

 takes place from without inwards through the membrane, and the drop 

 therefore grows continually in size, sometimes sending out branches as a result 

 slight currents in the fluid set up by accidental vibrations. Sugar intro- 

 uced into such a drop, although quickening its rate of growth,. does not pass 

 out into the surrounding copper sulphate solution, nor is there any passage 

 of copper sulphate inwards or potassium ferrocyanide outwards. Pfeffer con- 

 ceived the idea of depositing such a semi-permeable membrane within the 

 'erstices of a clay cell. Strengthened in this way, it is able to afford a 

 istance to pressure, and therefore to permit of the contained fluid reaching 

 full osmotic pressure. For this purpose a porous jar carefully cleansed 

 and containing a solution of sugar mixed with a little copper sulphate is 



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