OF ARTS AND SCIENCES. 417 



Investigations on Light and Heat, made and pubushed wholly oe in paet with Appropeiation 



FROM the Rumford Fund. 



XIX. 



BRIEF CONTRIBUTIONS FROM THE PHYSICAL LABORATORY 

 OF HARVARD COLLEGE, UNDER THE DIRECTION OF PRO- 

 FESSOR JOHN TROWBRIDGE. 



RELATION BETWEEN SUPERFICIAL ENERGY AND 

 THERMO-ELECTRICITY. 



By Charles Bingham Penrose. 



Communicated by Professor Trowbridge, June 11, 1884. 



When two media which do not mix are in contact, the particles near 

 the surface have more energy than similar particles in the interior of 

 the media. It is probable that this increase of energy is sensible only 

 within a distance of a thousandth of a millimeter fiom the surface. 



The result of this difference of energy is to render the surface of 

 contact of the media as small as possible. This is seen in Plateau's 

 experiment upon a mixture of oil in alcohol and water. 



When the area of the surface is increased in any way the surface 

 energy is increased, and work can be done in the contraction of the 

 surface. The surface behaves exactly as if a tension equal in all di- 

 rections existed at every point of it. In discussing surface energy it 

 can therefore be considered in two ways. It can be considered as 

 part of the internal energy of the body; and in this case to the in- 

 ternal energy that is generally understood in thermodynamics must 

 be added a term which shall be proportional to the surface, and also 

 a function of the temperature. Or it may be introduced into the 

 equations of the external work : and in increasing the surftice it can 

 be said that work is done against superficial tension in the same way 

 that work is done against an external pressure. 



The numerical value of the surface energy per unit of area is equal 

 to that of the surface tension per unit of length. 



Since the suj^erficial tension depends not only upon the body itself, 



but also upon the substances in contact with the body, and since woik 



done by, or against, sujjerficial tension depends not merely upon the 



initial and final states of the body, but upon the manner of passing 



VOL. XX. (n. s. XII.) 27 



