502 SIR DAVID BREWSTER ON THE COLOURS OF THE SOAP-BUBBLE. 



colourless state, as shown in a former experiment, where, in a concave film, it 

 was collected into a double convex lens, in the centre of the black film. 



It is impossible to convey in language an adequate idea of the molecular 

 movements and the brilliant chromatic phenomena, exhibited on the soap bubble ; 

 and it is equally impossible for art to delineate them. The visible secretion of a 

 colourless fluid from a film less than the 12,000th of an inch in thickness — its 

 separation into portions of every possible colour — the quick passage of these 

 portions into bands of the different orders in Newton's scale — their ever- varying 

 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 the margin of the concave or convex films — and 

 their reabsorption by the film by means of mechanical diffusion — are facts consti- 

 tuting a system of visible molecular actions, of which we have no example, and 

 nothing even approaching to it in physics. 



The phenomena of colour, described in the preceding experiments, are more 

 various and beautiful than any I have ever witnessed, whether caused by re- 

 fraction, or by thin plates under the influence of common or polarised light. 

 The compound tints produced by ever- varying changes in the combination of the 

 differently-coloured molecules, have a brilliancy and a peculiarity of hue which 

 I have never before observed ; and I am persuaded that, if we could examine 

 them prismatically under the microscope, we should obtain remarkably banded 

 spectra. 



Owing to the small size of the soapy film, it is not easy to show these colours 

 to more than one individual, but it would not be difficult to exhibit them by the 

 magic lantern to the largest audience. 



V. On the Mode of Producing Plane and Curved Films, and Examining the Phenomena 



they Exhibit. 



In conducting experiments on the colours of the soap-bubble, it is of great 

 importance to be able to produce films perfectly plane, and films with various 

 degrees of convexity and concavity, to protect the colouring matter upon them 

 from currents of air, and get rid of the extraneous light by which the phenomena 

 may be obscured. 



1. Plane Films. — Plane films are often produced by dipping the rim of a 

 cylindrical wine-glass in the soap solution ; but they are frequently convex, and 

 sometimes concave, results which I cannot explain. 



Surfaces almost perfectly plane are invariably produced by using cylinders of 

 glass or metal, open at both ends. They are more perfect when the metallic rim 

 is smooth and accurately circular. 



