500 SIB, DAVID BREWSTER ON THE COLOURS OF THE SOAP-BUBBLE. 



wire rectangles crossed at right angles,* the four curved films in which it is 

 encased pour upon it their colouring matter in the form of tadpoles, and speedily 

 efface the horizontal bands which are formed upon it when held vertically. The 

 streams of colouring matter which ascend to the apex of the film, along its delicate 

 fluid rim, are seen to much greater advantage than in films adhering to a wine-glass 

 or any other solid body. They are not impeded in their ascent by capillary attrac- 

 tion, and, in general, the currents are in succession of the same colour and order. 



There seems to be no direct means of determining the nature of the colouring 

 matter thus active upon the surface of the soap-film, but there is one fact which 

 may prove useful in such an inquiry. 



When a convex film was stretched across a conical wine-glass, and had stood 

 ten minutes, it gave brilliant coloured rings of the first and second order. A lens, 

 with its lower side concave, was placed above the film, and happened to be very 

 near it. When the rings had increased to the third and fourth order, the film rose up, 

 as if by attraction, or more likely from a slight increase in the temperature of the 

 air within the glass, and broke upon the concave surface of the lens, leaving a 

 ring of fluid T37 inches in diameter, and within it a distinct colouring matter, 

 which continued visible for several hours. 



In order to see more distinctly the coloured rings thus embalmed, I covered the 

 concave side of the lens with black wax, so as to reflect as little light as possible, 

 and repeated the experiment. The film again rose and burst, and the colouring 

 matter remained upon the wax thirty-two hours. Even when the wax is at some 

 distance from the film, the experiment will always succeed by heating the air 

 within the wine-glass, and increasing the convexity of the film till it touches the 

 wax. The electrical excitation of the glass by friction will doubtless answer the 

 same purpose. Whatever, therefore, be the nature of the colouring matter left on 

 the wax, it cannot consist of a number of thin, plates of soapy water. 



These various phenomena may be finely seen and shown to others by placing 

 the film in a cone of divergent light. The transmitted beam will exhibit the 

 colourless fluid in the act of secretion from the film, rushing up in tadpole and 

 other forms from its circumference to its apex, while groups of circular portions 

 with dark edges and white centres descend from the upper part of the film. The 

 horizontal colour bands while thus forming, and after being formed, will be seen 

 magnified in the reflected beam, and may be farther magnified if we reflect them 

 from a convex film. 



In the formation and decay of the coloured bands, the portions of the coloured 

 matter, which I have called tadpoles, perform a curious part. Their movements, 

 whether we examine them singly, or when revolving in one or more masses, 

 while other parts of the film are at rest, are very extraordinary ; and their 

 mutual actions, and apparent vitality, are so unlike anything that has been 



* See the following paper. 



