CONDUCTIVITY 



151 



substances. This is in the way that the two different solutions, 

 separated by a semi-permeable surface, do not follow the known 

 laws of diffusion, but are altered in that certain substances in 

 contrast to their rapidity of diffusion pass through the membrane 

 or are prevented from entering by the latter. This applies like- 

 wise to the two kinds of ions, which are dissociated in diluted 

 substances. If the surface exercises a selection in the way, 

 for instance, that the positive kations are allowed to pass through, 

 whilst the negative anions are held back, a difference of potential 

 must exist between the two. In this manner, wherever two differ- 

 ent solutions are separated from each other by a semi-permeable 

 surface, an opportunity occurs for the taking place of galvanic 

 currents. As we know, living protoplasm by reason of its colloi- 

 dal components possesses, in common with all colloidal sub- 

 stances, on its surface the properties of semi-permeable mem- 

 branes. Between the cell and the medium, therefore, there is 

 always the opportunity for the occurrence of differences of elec- 

 tric potential. But more. We likewise know that protoplasm itself 



0° o e> o o o o t 



Fig. 27. 



Scheme of the foam structure of living substance. A— In 

 undifferentiated protoplasm. B— In fibrillae protoplasm. 



