1890.J on Foam. 87 



A prettier form of the same experiment is due to Yan der 

 Mensbrugglie, who illustrated liquid tension by means of a film in 

 which he allowed to float a loop of fine silk, tied in a knot. As long 

 as the interior of the loop, as well as the exterior, is occupied by the 

 liquid film, it shows no tendency to take any particular shape : but 

 if, by insertion of, say, a bit of blotting paper, the film within the 

 loop be ruptured, then the tension of the exterior film is free to act, 

 and the thread flies instantaneously into the form of a circle, in conse- 

 quence of the tendency of the exterior surface to become as small as 

 possible. The exterior part is now occupied by the soap film, and the 

 interior is empty. Many other illustations of this property of liquids 

 might be given, but time does not permit. 



In the soap film, as in the films which constitute ordinary foam, 

 each thin layer of liquid has two surfaces ; each tends to contract ; 

 but in many cases we have only one such surface to consider, as when 

 a drop of rain falls through the air. Again, suppose that we have 

 three materials in contact with one another, — water, oil, and air. 

 There are three kinds of surfaces separating the three materials, one 

 separating water and oil, another oil and air, and a third surface 

 separating the water from the air. These three surfaces all exert a 

 tension, and the shape of the mass of oil depends upon the relative 

 magnitudes of the tensions. As I have drawn it here (Fig. 1), it is 

 implied that the tension of the water-air surface is less than the sum 

 of the other two tensions — those of the water-oil surface and the air- 

 oil surface; because the two latter acting obliquely balance the 

 former. It is only under such conditions that the equilibrium of the 

 three materials as there drawn in contact with one another is possible. 

 If the tension of the surface separating water and air exceeded the 

 sum of the other two, then the equilibrium as depicted would be 

 impossible. The water-air tension, being greater, would assert its 

 superiority by drawing out the edge of the lens, and the oil would 

 tend to spread itself more and more over the surface. 



Fig. 1. 



AIR 



WATER 



And that is what really happens. Accurate measurements made 

 by Quincke and others, show that the surface tension separating 

 water and air, is really greater than the sum of the two others. So 

 oil does tend to sj)read upon a surface of water and air. That this is 

 the fact, we can prove by a simple experiment. At the feet of our 

 chairman, our Honorary Secretary, is a large dish, containing water 

 which at present is tolerably clean. In order to see what may happen 

 to the surface of the water, it is dusted over with fine sulphur powder, 

 and illuminated with the electric light. If I place on the surface 



