The Causes of Osmotic Pressure. 493 



processes within the metal. Since, however, the viscosity of 

 steel decreases in marked degree from soft to hard * one would 

 first have to show that the viscosity in cold hard steel is 

 greater than the viscosity in the hot tempered steel, in any 

 comparison. 



Brown University, Providence, U.S.A. 



LXI. The Causes of Osmotic Pressure and of the Simplicity of 

 the Laws of Dilute Solutions. By William Sutherland f. 



WHILE the new school of physical chemists has been busy 

 pushing on the theory of dilute solutions to all its 

 available conclusions, physicists have been more concerned 

 about the secure establishment of the premises on which all 

 these conclusions are based. The feeling of physicists has 

 been well voiced by Fitz Gerald in the last Faraday lecture 

 at the Chemical Society of London, and Poynting's paper on 

 "Osmotic Pressure" (Phil. Mag. 1896, October) is one 

 attempt at securing a sound physical foundation for the theory 

 of solutions. The present paper is another. The reasons 

 usually assigned for the remarkable fact that the laws of the 

 osmotic pressure of a solute in a solvent are the same as if the 

 solvent were annihilated and the solute left as a perfect gas, 

 are quite inadequate, because they would apply to any selected 

 lot of the molecules of the solvent, and therefore to all the 

 molecules, with the result that the laws of a perfect gas would 

 have to hold for all fluids down to and in the liquid state. 

 The essence of the whole matter lies in this : — Why should 

 the laws of gases hold for the osmotic pressure of the solute, 

 when the solvent is in a condition involving enormous 

 departure from the laws of gases ? The answer is to be found 

 in a closer study of the semipermeable membrane. If we seek 

 to picture to ourselves how a membrane allows water mole- 

 cules to pass, but not sugar molecules, our simplest conception 

 of its structure is that of a mesh, amidst the threads of which 

 the water molecules are packed in such a manner as to give 

 way before one another almost as in ordinary water, while 

 the sugar molecules are held back by the mesh. Thus the 

 mesh forms a solid or quasi-solid framework through which 

 water can pass with high viscous resistance, while the sugar 

 molecule is absolutely blocked. Now if the framework turns 

 back the sugar molecules, it must take the force of their 



* Barus, Phil. Mag. [5], xxvi. p. 183 (1888) ; cf. < Nature,' xli. p. 3G9 

 (1890). 



t Communicated by the Author. 

 Phil. May. S. 5. Vol. 14. No. 271. Dec. 1897. 2 



