ELECTRICAL CHANGES IN LIVING TISSUES 193 

 where UjV Jt u 2 v 2 , are the velocities of the corresponding ions. We 

 assume that the concentrations of the two solutions are identical. 

 Now it is evident that by making w 2 and % very small, the expres- 

 sion log. 10 may be made to attain anv quantity, and in the 



V+?1 



same way by making u t + v 2 infinitesimally small the electromotive 



force of the combination will also become correspondingly small. The 



thickness of the membrane does not come into the formula, so that 



membranes of microscopic or even ultramicroscopic 



thickness, which we have seen reason to assume as 



present in and around cells and their parts, could 



perform all the functions required of the hypothetical 



membrane in the above example. This is also the case 



when Vj is the same as V 2 that is to say, there is a 



common anion or a common cation on the two sides 



of the membrane. 



It must be remembered that the passage of a 

 current through a membrane impermeable to one or 

 other ion in the surrounding fluid will cause an accumu- 

 lation of the ion at the surface of the membrane, so 

 that this will become polarised. Such an accumu- 

 lation at any surface will naturally alter the properties 

 of the surface, including its surface tension. The 

 construction of the capillary electrometer depends on 

 this fact. When mercury is in contact with dilute 

 acid or mercuric sulphate solution it takes a positive 

 charge from the fluid, and the state of stress at the 

 surface of contact between the mercury and the negatively charged 

 fluid diminishes the surface tension of the mercury. If the mercury 

 be in the form of a drop in a tube drawn out to a capillary, 

 the mercury will run down the capillary and the drop will be 

 deformed until the surface tension tending to pull the mercury 

 into a spherical globule is just equal to the force of gravity tending 

 to make the mercury run out through the end of the capillary 

 (Fig. 32). If the mercury be immersed in sulphuric acid it will descend 

 to a lower level in the capillary owing to the diminution of its surface 

 tension. If now the acid and the mercury be connected with a source 

 of current so as to charge the mercury negatively, the effect will be 

 to diminish the charge previously taken up by the mercury. The 

 state of tension at the contact with the acid is therefore diminished, 

 the surface tension is increased, and the mercury withdraws itself 

 from the point of the capillary. If, however, the^mercury be con- 

 nected with the positive pole, its charge will be increased and its 

 surface tension correspondingly diminished, so that the meniscus 



13 



FIG. 32 



