OSMOTIC PRESSURE AND THE THEORY OF 

 SOLUTION. 



By Prof. W. A. D. Rudge, M.A. 



Few of the modern physico-chemical theories have proved 

 of such far-reaching importance as the theory of osmotic pres- 

 sure as developed from the original observations of Traube and 

 Pfeffer. by Van 't Hoff, Arrhenius and others too numerous to 

 mention. The theory had to make its way in the face of 

 vigorous opposition, and even at the present time there are 

 found some disposed to reject it, especially as it has been, 

 owing perhaps to lack of perfect experimental methods, found 

 to fail in certain cases, i.e., in strong solution. We shall now 

 enquire what this theory is, and why it has been such a fertile 

 source of advancement. The theory is woven very intimately 

 with that of electrolytic dissociation, in fact one may be said 

 to depend upon the other, and the connection will be tacitly 

 assumed in this paper. Let a solid substance be placed in a 

 vessel containing a liquid in which the substance can dissolve, 

 and further, let the amount of solid be small compared with 

 that of the liquid. The solid will dissolve, and in the course of 

 time will diffuse throughout the whole of the solution. Now 

 what is the condition of the solid in the solution ? Is it the 

 same as in the solid state? The answer is " no " in the case 

 of most salts. For example, a lump of rock salt does not con- 

 duct electricity, is, in fact, an insulator. Water is also prac- 

 tically a non-conductor, but water with only ^hr of its weight 

 of salt in solution conducts fairly w'ell, so that clearly the salt 

 in solution is in a different physical state from solid salt. What 

 is true for common salt in water is true for other salts dis- 

 solved in other solvents. The particles of solid salt are now 

 changed into atomic or molecular groups or " ions," which 

 have great freedom of movement through the solution, and 

 the salt is said to be " ionised." As mentioned above, a solid 

 substance placed in liquid in which it can dissolve will not re- 

 main in the bottom of the vessel, but will slowly diffuse through 

 it against the force of gravity, and there must be some 

 impelling force producing this motion. Now imagine 

 a quantity of pure water is confined in a closed vessel. 

 At all points on the same level below the upper sur- 

 face the pressure will be the same, which will be true 

 also for a mass of gas confined in a similar vessel, 

 but neglecting the weight of gas, the pressure will be the same 

 at all parts of the vessel. Imagine again a similar vessel com- 

 pletely exhausted, but into which a small amount of gaseous 

 matter may be introduced. This matter expands, filling the 

 whole of the space, and a pressure will be developed upon the 

 walls of the vessel of a magnitude proportional to the quantity 

 of matter introduced. All this is readily understood, of course, 

 from the ordinary laws of gases. To take a concrete example, 

 let the volume of the vessel be 22'32 litres, and the amount of 

 gas, say hydrogen, introduced be equal to the molecular weight, 

 i.e.. 2 grams, then the pressure inside the vessel will at o°C. 

 be raised to one atmosphere, and the pressure produced by the 

 gram molecular weight of iviy ga.<!. simple or compound, will 



