The particles on every side of the individual particle attract it, and the attraction of opposite particles on every side tends to neutralize each other, so that the individual particle has almost perfect 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 Fig. 5. is potential energy, and is inherent in the surface particles in virtue of their position. If we consider an oily film to be spread over the surface of a body of water, it will appear that the particles near the surfaces which separate the oil from the water and from the air must have greater energy than those in the interior 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 surface Fig. 6. 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 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