36 GENERAL PROPERTIES OF LIVING TISSUES 



the difference in potential. The action of the 

 instrument will be more clear from the follow- 

 ing experiments. 



Surface Tension altered by Electrical Energy. 

 In a small porcelain evaporating dish place a 

 globule of mercury about one inch. in diameter. 



The cohesion of the mercury is stronger than 

 the attraction between the mercury and porce- 

 lain, the mercury does not " wet " the porcelain. 

 The free surface of the mercury is curved and 

 not plane, as it would be were the molecules 

 acted upon by the force of gravity alone. Obvi- 

 ously the spreading of the mercury is resisted by 

 some force that strives to make the drop spher- 

 ical, i. e. to make the surface as small as possible. 



This force is called the surface tension. It is 

 the attraction which the molecules beneath the 

 surface exert on the side of the surface layer 

 next them. The form of the drop is the result 

 of the equilibrium between these opposing forces 

 (Thomas Young, 1804). 



Cover the mercury one centimetre deep with 

 5 per cent sulphuric acid. Note carefully the 

 degree of convexity. Add a trace of potassium 

 bichromate. The drop will flatten slightly. 



When a metal is placed in an electrolyte, a 

 difference of potential is created at the surfaces 

 in contact. If the metal is positive compared 



