20 THE THEORY OF IONS 



tion to the molecules + ions in the solution. Sugar 

 is a non-electrolyte, and 0-5 per cent, solution exerts 

 exactly half the pressure of a 1 per cent, solution. 

 But NaCl is an electrolyte, and a 0-5 per cent, 

 solution exerts more than half the pressure of a 

 1 per cent, solution, because the dissociation is 

 greater and there are relatively more ions in the 

 weaker than in the stronger solution. 

 ' During diffusion, it is found that the presence of 

 a dividing medium or septum influences the rate of 

 diffusion and the velocity of the ions. The rate of 

 diffusion of various gases through porous septa is 

 nearly inversely proportional to the square root of the 

 density of the gases ; thus H passes through porous 

 septa four times as quickly as 0, and nearly three 

 times as quickly as steam. * Many experiments have 

 been performed which show that septa influence 

 the velocity of the transmission of ions. The re- 

 lative velocity of cations is in all cases less when a 

 dividing membrane is used than when no mem- 

 brane is used ; but the difference is greatest when 

 the septum consists of an animal membrane, and is 

 smaller or comparatively negligible when the 

 septum consists of parchment paper or porous clay.f 

 Bein determined the rate of transmission of ions 

 through clay, parchment paper, gold-beater's skin, 

 and fish-bladder of H, Na, Li, Ca, and Cd in the 

 form of chlorides, and in every instance the rate of 

 transmission of cations through gold-beater's skin 

 and fish-bladder was diminished. J Absorption 



* Watts' " Diet. Chem." 



t Bein : Zeit. Physikal-Chem., 1899, xxviii., 439. 

 LOG. cit. 



