OSMOTIC PRESSURE. 443 



produced by the motion of the ions will act at the limiting sur- 

 faces of the liquid, thus tending to increase the volume. If 

 the volume increases the pressure will diminish. By the aid 

 of a semi-permeable membrane, used to enclose the liquid, and 

 surrounded by pure water, the volume of the solution inside 

 the membrane will increase until some condition of equilibrium 

 is reached by the passage of water through the membrane into 

 the solution. The most satisfactory semi-permeable membrane 

 is composed of copper-ferrocyanide, built up in the walls of 

 a porous vessel by standing the vessel in a solution of copper 

 sulphate, and filling it up with potassium ferrocyanide solution. 

 Diffusion of the two salts will go on, and the pores of the 

 vessel will be filled up with a continuous layer of copper fer- 

 rocyanide. Wash the vessel free from the salts, attach to it 

 some kind of pressure gauge, fill it up with distilled w^ater. 

 hermetically seal it. and stand it in a large vessel of water. 



The pressure gauge will show no increase of pressure inside 

 the vessel. Xow remove the water from inside the membrane, 

 and substitute a solution of sugar, placing it once more in the 

 water. The gauge will now indicate an increased pressure, 

 which will continue to increase until some definite maximum 

 pressure is attained. The dissolved salt exerts a pressure upon 

 the liquid, tending to stretch it. At first sight it would appear 

 that relief from this pressure would be obtained by something- 

 passing out of the vessel through the semi-permeable mem- 

 brane to the water outside, but this is impossible, because (i) 

 if the w^ater passed out the solution would become stronger, the 

 pressure therefore increasing, and (2) the semi-permeable mem- 

 brane will not aJlozv tJie dissoli'ed substance to pass. Water, 

 however, may flow in, and as it enters the volume of the solu- 

 tion increases, and the pressure falls. One has to italicise 

 *' falls." because the pressure, as indicated by the gauge, ap- 

 pears to rise, and finally equilibrium is obtained when the pres- 

 sure, as shown by the gauge, is balanced by the internal 

 pressure of the dissolved substance on the volume of the solu- 

 tion now present. 



This pressure may reach very high values, in fact, the actual 

 pressure attained is practically limited by the strength of the 

 membrane, which l^ecomes ruptured if submitted to too great 

 a strain. 



The direct measurement of osmotic pressure has always been 

 a matter of some experimental difiiculty. Satisfactory 

 results had been obtained only with dilute solutions, and 

 the theory was (and still is) considered by some as untenable : 

 but recent researches by the Earl of Berkeley, Burton, and 

 Vegard have shown that with properly prepared membranes 

 equally good results may be obtained with strong solutions. 



The theory of osmotic pressure, however, shows us that its 

 value may be determined in an indirect manner, viz., by ob- 

 serving (i) the freezing point of a solution of a solid, as com- 

 pared with that of a pure solvent; (2) the rise in the boiling 

 point of a solution, and (3) the electrical properties of the solu- 

 tion. 



