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XXXIV. — On the Figures of Equilibrium in Liquid Films. 

 By Sir David Brewster, K.H., F.R.S. (Plates XXXIV., XXXV., XXXVI.) 



(Read 4th February 1867.) 



In repeating some of the beautiful experiments of Professor Plateau, on the 

 Equilibrium of Liquid Films, contained in seven Memoirs, published in the 

 " Transactions of the Royal Belgic Academy,"* and in prosecuting my experi- 

 ments on the colours of the soap-bubble, I observed several new phenomena 

 which may have escaped the notice of the Belgian philosopher. 



In plunging a wire cube in a solution of soap, and lifting it up vertically, Pro- 

 fessor Plateau found that there was formed within it a polyhedron, as shown in 

 Plate XXXIV. Fig. 1, consisting of twelve similar liquid films adhering by capil- 

 lary attraction to the twelve wires which compose the cube, and a small quadran- 

 gular film suspended in the middle of them. In many cases M. Plateau found 

 that the vertical quadrangular film was often horizontal, as in Fig. 2 ; and M. 

 Van Rees discovered, that by blowing very lightly upon one of its sides it was 

 reduced to a simple line, and then reproduced in a horizontal position, from 

 which it could be blown again into a vertical position, as in Fig. 1. 



Considering that a system formed of twelve films joined at the centre of the 

 cube, as shown in Fig. 3, would, on account of its perfect symmetry, be a system 

 of equilibrium, M. Plateau was surprised to find that it could not be produced 

 and rendered stable, without introducing something solid into the system. To 

 do this, he stretched a fine iron wire from one summit of the wire cube to the 

 opposite summit, and having, after immersion, withdrawn it from the glycerine 

 solution, there was at the central point a small quadrangular film which slowly 

 got less and less, till the system in Fig. 3 was produced. M. Plateau subse- 

 quently found that this system was permanent, when a small drop of the glyce- 

 rine solution was accidentally retained at the centre of the system. As this drop 

 could exist only as a sphere, it is not easy to understand how the twelve trian- 

 gular films could be united with it at their apex, so as to produce a state of 

 stable equilibrium. 



In repeating these interesting experiments I found that the polyhedron, with 

 a horizontal quadrangular film, Fig. 2, was always produced in the solution I 

 employed, and by blowing through a narrow tube, either upon the edge or the 

 middle of it, it was changed, as Van Rees found, into a vertical quadrangular 

 film, as in Fig.l; but by a longer blast, it passed into the unstable system of Fig. 3, 



* Memoires de l'Academie Belgique, tomes xvi. xxiii. xxx. xxxi. xxxiii. and xxxiv. 

 VOL. XXIV. PART III. 6 X 



