THE ENERGY OF MOLECULES IN SOLUTION |j;, 



molecules will cause them to wander from the lower to the upper part t 

 this process of diffusion will cease only when the concentration haTbecome 

 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 

 involves the performance of an amount of work determined by 

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

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

 less than the osmotic pressure exerted by the sugar solution, 

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

 solution 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 that represented in the diagram, for the performance 

 of work. 



THE MEASUREMENT OF THE OSMOTIC PRESSURE. By a method 

 differing 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 t 

 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, 

 duced into such a drop, although quickening its rate of growth, c 

 out into the surrounding copper sulphate solution, nor is there a 

 of copper sulphate inwards or potassium ferrocyanide outwn n Is. 

 ceived the idea of depositing such a semi-permeable membra 

 interstices of a clay cell. Strengthened in this way, it 11 

 resistance to pressure, and therefore to permit of the contained fluid rea 



