116 SIR DAVID BREWSTER ON THE MOTION, EQUILIBRIUM, 



ing a small bubble in the centre of the polyhedron. A precisely similar result 

 may be obtained with the similar system within the cone, and that by either of the 

 same methods. The result of causing this system to ascend the cone is strikingly 

 beautiful ; the cube becomes narrowed above, assuming the form of a truncated 

 pyramid ; and if caused still further to ascend, the horizontal film at its apex 

 entirely disappears, and a perfect pyramid is produced, resembling that shown 

 in Plate xxxiv. Fig. 10, of vol. xxiv. of " The Transactions."* 



At the same instant that this takes place, the four original bubbles become 

 united above the pyramid in a vertical straight line, but this system being 

 unstable, the line quickly passes into a vertical plane film. 



As the system continues still further to ascend the cone, it generally settles 

 into one or other of the forms already described, i.e., two, three, or four similar 

 hollow figures united by one or more plane vertical films. At any time before 

 the pyramid disappears, we can again reduce the system to its original form by 

 blowing air in at MN. This experiment is best made with a cone whose 

 vertical angle is more obtuse than that described. 



III. On the llotions and Figures of Equilibrium of Films ivithin Double Hollow Cones. 

 In these experiments a glass vessel was used of the form of that in the 

 annexed diagram, consisting of two cones united by their bases. The vessel, as 

 in the last case, can be closed or opened by a plug fitting at 

 MN. When MN is open, and the vessel is dipped into the 

 soap solution, it gives, when raised, a single plane film at EF. 

 When the vessel is dipped so that the liquid rises in it a little 

 below CD, and MN is then entirely or nearly closed, we obtain, 

 on raising the vessel so that the liquid runs out, a convex film 

 below CD, and a concave one at EF. 



If we make the same experiment with the vessel dipped so 

 deep that the liquid rises above CZ>, we obtain a variety of 

 regular or irregular systems of films, which, when destroyed, 

 often leave a fine concave film above CD. On opening MN, this concave film 

 rises to A B, the rings or bands of colour rising to higher orders by the thicken- 

 ing of the film. This concave film is sometimes formed along with and above the 

 regular or irregular systems of films, and separate from them. 



When a film is formed exactly at CD, it is fixed, and moves neither towards 

 A B nor EF; but if it be made slightly to advance towards A B, it will rise to 

 A B, or if made to advance in the direction of EF, it will fall to EF, and there 

 become stationary. Regular binary, ternary, or quaternary systems, similar to 

 those already described as formed within the single cone (the quaternary system 



* The so-called cube is of course more or less a truncated pyramid from the first, owing to 

 the conical form of the vessel used. 



