174 



PHYSIOLOGY 



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 sul- 

 phuric 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. 

 at 9^ The state of tension at the contact with the acid is therefore 

 I diminished, the surface tension is increased, and the mercury 

 H withdraws itself from the point of the capillary. If, however. 

 \ W fl the mercury be connected with the positive pole, its charge 

 \^^m/ w iU b e increased and its surface tension correspondingly 

 ^^f/ diminished, so that the meniscus will move towards the 

 \ W/ point of the capillary. The movement of the meniscus to or 



1 ff I away from the point may thus be used, as in the capillary 



UU electrometer, to show the direction and amount of any 



FIG. 32. moderate electric change occurring in a tissue, two points 

 of which are connected with the mercury and the acid re- 

 spectively. It is possible that this electrical alteration of surface tension 

 may be a determining factor in many of the phenomena of movement 

 observed in the animal body. We shall have occasion to discuss this 

 question more fully when endeavouring to account for the ultimate nature 

 of muscular contraction. 



