GENERAL PHYSIOLOGY OF MUSCLE AND NERVE TISSUE 67 



FIG. 40 



upon a horizontal plane its shape will be modified by three forces : 

 (1) cohesion, (2) adhesion, (3) gravitation. In the case of the globule 

 of water on an oiled surface, or of mercury on a horizontal glass 

 plane, adhesion is practically nil, thus leaving the two factors, cohesion 

 and gravitation. 



Cohesion tends to draw all the molecules toward a common center 

 and thus brings the individual molecules of the surface into a con- 

 dition of lateral tension. This condition is tech- 

 nically called surface tension. The greater the 

 preponderance of cohesion over the other forces 

 acting upon the liquid the greater the surface 

 tension. 



It is surface tension which gives to the mercury 

 in the glass tube a convex meniscus, and keeps it 

 from flowing through a fine capillary of glass. 



It must be evident that the relation between 

 the mercury and the glass (adhesion) does not 

 vary. If the position of the meniscus varies it 

 must be through a change in one or both of the 

 other forces mentioned above. 



Gravitation measured by the weight of the 

 column of mercury may vary by changing the 

 height of the column of mercury. Through this 

 variation the meniscus may be made to take any 

 desired position. 



Experiment has shown that the passage of an 

 electric current through the column of mercury 

 into sulphuric acid modifies the surface tension 

 of the mercury and thus changes its position. As 

 the modification of surface tension varies propor- 

 tionately with the strength of the electric potential, 

 one may measure this strength by noting the 

 distance through which the meniscus moves. 



The observation of the meniscus must be made with a microscope, 

 using the low power. Note that in the instrument (see Fig. 40) the 

 wooden back that supports the instrument is cut away (at 0) near 

 the capillary in order to permit the microscopic observation of the 

 meniscus to be made with transmitted light. 



(b) The Method of Using the Capillary Electrometer. 



To adjust the instrument for use clean the capillary absolutely 

 clean through the use of 20 per cent. H 2 SO 4 c. p. and distilled water. 

 Pour into the tube enough mercury to bring the meniscus to the 

 middle of the finest portion of the capillary. Adjust the parts of 

 the electrometer the pressure bulb P, the manometer ra, and the 



Capillary electrometer. 

 (Description in tezt.) 



