SIR DAVID BREWSTER ON THE COLOURS OF THE SOAP-BUBBLE. 499 



innumerable configurations which we have described, and finally return into 

 their original colourless condition, are problems of great interest and diffi- 

 culty. 



When we examine the surface of a solution of soap, or of the glycerine mixture, 

 in a vessel however shallow, no colour or colouring matter appears upon it. It 

 reflects the images of objects, like the surface of water or glass ; but as soon as it 

 is blown into a bubble or formed into a film, its surface becomes for a time uneven, 

 and reflects objects imperfectly, in consequence of its being covered with round- 

 edged waves or tadpole portions of a colourless fluid moving from the margin of 

 the film. When the film is held vertically, these portions rush chiefly to the cir- 

 cumference of the film, and, rising in streams of considerable velocity, give out 

 molecules of different colours, and consequently of different sizes, to form the 

 horizontal bands, carrying the smallest to the apex, to form the black of the first 

 order, and those of a greater size to the bands of the higher orders. 



When the colour molecules have thus arranged themselves, they may be scat- 

 tered, as already mentioned, by blowing, or by pouring upon them some of the 

 soapy solution, or even by brushing them from their place by a feather wet with 

 the solution. Thus scattered over the film, they are singularly mixed together, 

 so as to produce compound tints ; but when the film rests in a vertical or hori- 

 zontal position, they re-arrange themselves under the influence of gravity, taking 

 their place in bands or circles as above described. 



The mode in which these changes take place, and by which the horizontal bands 

 are broken up, as shown in Fig. 14, is well seen when a portion of one or more bub- 

 bles adhere to the margin of the film, as represented in Fig. 15. When the bands of 

 three or four orders are produced in this film, held vertically, the rush of coloured 

 molecules, in the tadpole form, from the margin mn of the bubble B, disturbs, 

 or effaces, as it were, the regular bands ; and when the bubble is burst, or bursts 

 spontaneously, it leaves a film behind it, enlarging the original film, and scattering 

 all its colouring matter over the enlarged film. When there is a series of bubbles 

 round the margin of the film, the effect of their bursting, or being burst, in succes- 

 sion upon the successively enlarged film is very beautiful. In one of these expe- 

 riments the accidentally symmetrical position of four equal bubbles round a per- 

 fectly square central film, as shown in Fig. 16, was so remarkable as to deserve 

 being noticed. On all these bubbles there were coloured rings, or rather curved 

 bands, the colouring matter of which descended into the line of junction of the 

 bubbles with the film, and rose to obliterate or disturb its horizontal bands. 



In the preceding experiments the soap-film was fixed by capillary attraction to 

 the rim of a wine-glass ; but the general phenomena, and especially the formation 

 and breaking up of the coloured bands, may be best seen upon plane films which 

 are surrounded with other plane films, as in the figures of equilibrium discovered 

 by Professor Plateau. When an elliptical plane film is formed between two 



VOL. XXIV. PART III. 6 T 



