WHY A FILM OF OIL CAN CALM THE SEA. 499 



The particles on every side of the individual particle attract it, 

 and the attraction of opposite particles on every side tends to neu- 

 tralize each other, so that the individual particle has almost per- 

 fect mobility. The surface particles, however, inasmuch as all the 

 rest of the fluid is below them, are drawn inward toward the mass 

 of the fluid, and a certain tension is produced. This tension 

 is potential energy, 

 and is inherent in 

 the surface particles 

 in virtue of their 

 position. If we con- 

 sider an oily film to 



Taw 



Tow 



be spread over the 

 surface of a body of 

 water, it will appear 

 that the particles 

 near the surfaces 

 which separate the FIG. 5. 



oil from the water 



and from the air must have greater energy than those in the in- 

 terior of the film. The excess of energy due to this cause will be 

 proportional to the area of the surface of separation. When this 

 area is increased in any way, work must be done ; and when it is 

 allowed to contract, it does work upon other bodies. Hence it 

 acts like a stretched sheet of India rubber, and exerts a tension of 

 the same kind. 



In the above figure, which represents an exaggerated picture of 

 a layer of oil on the surface of a body of water, let Taw represent 

 the superficial tension of the surface separating air from water ; 

 let Tao represent the superficial tension of the surface separating 

 air from oil ; let Tow represent the superficial tension of the sur- 

 face separating oil from water ; and let P be a point 

 of the line forming the common intersection of the 

 surfaces separating the air, oil, and water. For the 

 equilibrium of these three media, the three tensions 

 Taw, Tao, and Tow must be in equilibrium along the 

 line of common intersection, and since these tensions 

 P IO 6 have been measured and are known, the angles which 

 their directions make with one another can be easily 

 determined ; for, by constructing a triangle, ABC, having sides 

 proportional to these tensions, the exterior angles will be equal to 

 the angles formed by the three surfaces of separation which meet 

 in a line. But it is not always possible to construct a triangle 

 with three given lines as its sides. If one of the lines is greater in 

 length than the sum of the lengths of the other two, the triangle 

 is impossible. For the same reason, if any one of the superficial 



