78 



SCIENCE 



[N. S. Vol. LIII. No. 1361 



taneously in opposite directions through the 

 membrane and no change in volume will 

 occur. When, however, the same experiment 

 is mad© with two non-ideal solutions contain- 

 ing equal num.bers of molecules in equal 

 volume, the result is different. As Tinker 

 has demonstrated mathematically, in this case 

 the flow of water must be from the solution 

 having the lower intrinsic pressure and lower 

 surface tension to the solution with higher 

 intrinsic pressure and higher surface tension. 

 Tliis is what Traube claims, and his theory 

 explains therefore, as Tinker jwints out, the 

 deviations from the gas law in the case of 

 non-ideal solutions, but it does not prove that 

 the gas law of osmotic flow does not hold in 

 the case of ideal solutions and Traube's theory 

 can not therefore replace van't Hoff's theory. 



There is a second group of forces not taken 

 into consideration in van't Hoff's law, namely 

 the influence of the chemical nature of the 

 membrane on the solvent. These forces be- 

 come noticeable when the membrane sepa- 

 rating the solution from the pure solvent is 

 not strictly semipermeable. When water is in 

 contact with a membrane it undergoes as a 

 rule an electrification and this electrification 

 of the particles of water plays a great role 

 in the rate of the osmotic flow when the 

 solution into which the water diffuses is an 

 electrolyte. 



The assimiption that water diffusing 

 through a membrane is as a rule, electrified, 

 is justified by a large number of observations. 

 Quincke demonstrated that when water is 

 pressed tlirough capillary tubes it is found 

 to be electrically charged (the sign of charge 

 being more frequently positive) ; while the 

 tube has the opposite sign of charge, e.g., 

 negative, when the water is positively charged. 

 When two solutions of weak electrolytes are 

 separated by a membrane (which may be con- 

 sidered as a system of irregular capillary 

 tubes) an electric current causes water to 

 migrate to one of the two poles, according to 

 the sign of its charge. By this method of 

 so-called electrical endosmose it can be shown 



that water diffuses through collodion mem- 

 branes in the form of positively charged par- 

 ticles. Collodion bags, cast in the shape of 

 Erlenmeyer flasks, are filled with a weak and 

 neutral solution of an electrolyte, e.g., 

 M/256 Na^SO^, and dipped into a beaker 

 filled with the same solution of lf/256 

 !N"a„SO^. The opening of the collodion bag 

 is closed with a rubber stopper perforated by 

 a glass tube serving as a manometer. When 

 a platinmn wire, forming the negative elec- 

 trode of a constant current, is put through 

 the glass tube into the collodion bag while 

 the other pole of the battery dips into the 

 outside solution, the liquid in the glass tube 

 rises rapidly with the potential gradient be- 

 tween the two electrodes. The water there- 

 fore migrates through the collodion membrane 

 in the form of positively charged particles. 

 The writer has made a number of experi- 

 ments^ concerning the osmotic flow through 

 collodion membranes, and it is the purpose of 

 this address to give a brief survey of the 

 results. 



m 

 When a collodion bag is filled with a solu- 

 tion of a crystalloid, e.g., sugar or salt, and 

 dipped into a beaker containing pure water, 

 the pure water will diffuse into the solution 

 and the level of liquid in the capillary glass 

 tube serving as a manometer will rise. At 

 the same time particles of the solute will 

 diffuse out of the bag (except when the solute 

 is a protein solution or a solution of some 

 other colloid). The concentration of a crys- 

 talloid solute inside the collodion bag will 

 therefore become constantly smaller until 

 finally the solution is identical on both sides 

 of the membrane. Ifevertheless the relative 

 force with which a given solution inside the 

 collodion bag " attracts " the pure water into 

 which the bag is dipped can be measured by 

 the initial rise in the level of water in the 

 manometer, before the concentration of the 

 solution has had time to diminish to any 

 great extent through diffusion. Since in the 



2Loeb, J., J. Gen. Physiol., 1918-19, I., 717; 

 1919-20, II., 87, 173, 273, 387, 563, 659, 673. 



