138 



PHYSIOLOGY 



the fact that the molecules of sugar pass through it only with diffi- 

 culty, and therefore in their passage outwards towards the confines 

 of the water exert a pressure on the walls, driving them apart and so 

 causing a distension of the bladder. It is impossible, however, by this 

 means to obtain the full osmotic pressure due to the pressure exerted 

 by the sugar molecules, since the bladder wall itself is not absolutely 

 impermeable to sugar. If we imagine the sugar solution confined in a 

 cylinder and covered with a layer of distilled water, the movement of 

 the sugar molecules will cause them to wander from the lower to the 

 upper part, and this process of diffusion will cease only when the 

 concentration has become the same in all parts of the solution. Sup- 

 posing, 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, PfefTer 

 succeeded directly in measuring the osmotic pressure of certain solu- 

 tions. For this purpose Pfeffer took advantage of the fact, discovered 

 by Traube, that various precipitates, if deposited in the form of mem- 

 branes, 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 ferro- 

 cyanide suspended to a glass rod be introduced carefully into a more 

 dilute solution of copper sulphate, it will be observed that at the 



FIG. 20= 



