6 REPORT 1867. 



Light. 



On the Colours of the Soap-Biilhle. 

 By Sir Datid Bkewstee, K.H., LL.D., F.B.S., 4'c. 



The colours of the soap-bubble have been the subject of frequent obsenntiou 

 since the time of Boyle, Hook, and Newton, and they have been invariably ascribed 

 " not to any colour' in the medium itself in which they are formed, or on whose 

 surfaces they appear, but solely to its greater or less thickness." The author of 

 this paper had been led to doubt the correctness of this opinion, and while re- 

 peating the beautiful experiments of Professor Plateau " On the EquiUbrium of 

 Liquid Films," he was led to discover the true cause of these colours, whether they 

 are observed on the soap-bubble or on plane, convex, and concave fihus sti-etched 

 across the mouths of closed or open vessels. 



The paper, which is illustrated with numerous coloured drawings, is divided into 

 five parts. 



1. On the phenomena of colour in a vertical plane film. 



2. On the production of revolving systems of coloured rings on the soap-film. 



3. On the form and movements of the bands and rings on con-\-ex and concave 

 films. 



4. On the phenomena prodiiced by different solutions. 



5. On the origin and development of the colours of the soap-bubble. 



In these sections the author has sliONvn that the colouring-matter of the soap- 

 bubble is secreted from the soap-solution v.-hen reduced to the state of a film ; 

 that it rises to the highest point of the film in colourless portions, in the form of 

 tadpoles, which pass into molecules in every possible order of colour, and then take 

 their proper place in the coloui-ed bands ; that these bauds move over the surface 

 of the film under the influence of gravity, and may be blown into fragments or into 

 molecules of all colours, or e^-en recombined with the fihn ; that they may be blo-^m 

 into two systems of coloured i-ings, the one revohing from light to left, and the 

 other froni left to right ; and that under the the influence of the centrifugal force, 

 these molecules are carried into their place in Ne^\i;on's scale — those of the first 

 orders going to the centre of the ringp, and followed by those of higher orders that 

 happen to be in the film, when it is blown upon through a tube in the direction of 

 a diameter. 



" It is impossible," the author adds, " to convey in language an adequate idea of 

 the molecular movements, and the brilliant chromatic phenomena exhibited on the 

 soap-films, and it is equally impossible for art to delineate them. The ^-isible secre- 

 tion of a colourless fluid from a film less than the twelve thousandth of an inch in 

 thickness, — its se])aration into portions of every possible colour, — the quick passage 

 of these portions into bands of the different orders in Newton's scale, — their ever 

 vaiTing forms and hues when the bands either break up spontaneously, or are 

 forcibly broken up, — their conversion into revolving systems of coloured rings under 

 the influence of a centrifugal force, — their various motions when the film is at rest, 

 and protected from aerial currents, — their recombination into a colourless fluid 

 when driven to the centre or margin of concave and convex films, and their reab- 

 sorption by the film by means of mechanical diffusion, are facts constituting a 

 system of visible molecular actions, of which we have no example, and nothing 

 even approaching to it in Physics." 



On the Figures of Equilibriian of Liquid Films. 

 By Sir David Beeavsteb, K.H., LL.D., F.E.S., 4'c. 



In repeating some of the experiments of Professor Plateau, described in seven 

 interesting memoirs published in the Transactions of the Belgian Academ}-, and 

 in prosecuting his own experiments on the colours of the soap-bublile, the author 

 of this paper observed several new phenomena which may have escaped the notice 

 of the Belgian philosopher. 



Professor Plateau has described and dra"svn the beautiful systems of soap-films, 

 obtained by lifting from a soap-solution a cube made of wires about one ana a half 

 inch long. This system is a polyhedron, composed of twelve similar films stretch- 



