ELECTRICAL CHANGES IN LIVING TISSUES 



173 



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 con- 

 nected 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 connected with 

 the positive pole, its charge will be increased and its surface 

 tension correspondingly diminished, so that the meniscus 

 will move towards the point of the capillary. The move- 

 ment of the meniscus to or away from the point may thus be 

 used, as in the capillary electrometer, to show the direction 

 and amount of any moderate electric change occurring in a 

 tissue, two points of which are connected with the mercury 

 and the acid respectively. 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. 



FIG. 32. 



