PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 287 



substance will adsorb can be ascertained only by experimen- 

 tation. 



A similar speculation based on electrostatics has been advanced 

 by 0. Raber. The membrane is viewed not as a charged porous 

 mass but as a colloidal suspension the elements or particles of 

 which are charged and react with ions in the same way as do the 

 particles of a colloidal suspension when a salt is added. 



Surface Tension. — Czapek suggested that a substance must 

 lower the surface tension of protoplasm in order to enter a cell. 

 A high surface tension would present an interfacial membrane 

 made up of tightly packed molecules between which substances 

 could not pass readily. Low surface tension would mean a loose 

 arrangement of surface molecules and therefore a more per- 

 meable membrane. In order to lower the surface tension of 

 protoplasm, a substance must have a tension value less than that 

 of the protoplasm. The hypothesis met with drastic criticism 

 to which Czapek made the reply that it must collapse only when 

 one substance has been found which, in spite of a surface-tension 

 value below that of protoplasm, does not enter. 



Adsorption. — Otto Warburg discarded all hypotheses of 

 selective permeability for one of adsorption. He found that the 

 degree of toxicity of the methyl, ethyl, propyl series is much 

 more closely related to the adsorptive powers of these alcohols 

 than to their surface tension or lipoid solubility. Those alcohols 

 and other substances that are more strongly adsorbed to the 

 surface get in the more readily. 



The Emulsion H5rpothesis. — Clowes found a striking analogy 

 between the behavior of emulsions and the experimental per- 

 meability data of Osterhout. As a result, he developed an 

 ingenious hypothesis of membrane permeability based on the 

 behavior of emulsions. Osterhout had found that sodium chlo- 

 ride increases and calcium chloride (sometimes) decreases the 

 permeability of cells. Loeb had previously found an ''antagon- 

 ism" between sodium and calcium, that is to say, the two 

 elements when present in proper proportions prevent the toxic 

 effect of each other. This proportion is that which exists in 

 physiologically balanced solutions (sea water and blood). 

 Clowes found that sodium (the hydroxide) causes the formation of 

 emulsions of the oil-in-water type, while calcium (the chloride) 

 brings about reversal to the water-in-oil type. Sodium and 



