SIR DAVID BREWSTER ON THE COLOURS OF THE SOAP-BUBBLE. 493 



ing matter sometimes occurs in small spots and in dendritic forms, moving over a 

 lighter black portion. 



The development of the black band is very remarkable. At first, a slightly 

 dark shade appears at the apex of the film, increasing in darkness till a patch of 

 deep black rushes in with a brilliant margin of white silvery spots, which adhere 

 to it even when its lower edge becomes a straight line. These silvery spots, in a 

 very minute state, and obviously of the first order, sometimes form a sort of net- 

 work within the black space, and at other times dilute, as it were, the black with 

 white specks, seen only with the microscope. 



2. If, when the film has four or five bands upon it, as in Fig. 4, or indeed any 

 number, we make the glass turn round its axis as quickly as possible, the bands 

 will remain horizontal, as if under the influence of gravity, proving that they are 

 not produced by the subsidence and consequent thinning of the film, for which 

 there is not time, as the change of place in the film is almost instantaneous. 



In confirmation of this result, place the film in a horizontal position. The 

 bands, now bent or broken, will float in irregular shapes on its surface, the film 

 itself, which is a fixture, adhering by capillary attraction to the margin of the 

 wine-glass. 



These phenomena are seen with interesting variations, when the film is at 

 first placed at different angles with the horizon. When it is nearly horizontal, 

 the changes take place slowly, and the movements of the separate portions of 

 colouring matter, to which I have given the name of tadpoles, is very curious, the 

 whole film being sometimes covered by them without the trace of a band. The 

 black matter is produced very irregularly, and in detached portions, which ulti- 

 mately unite, chasing away all the other colours at the lowest part of the film. 

 While these changes are going on, there is a considerable deposition of aqueous 

 vapour on the lower surface of the watch-glass which covers the film. 



3. When the first, second, and third, &c, bands are floating about, like oil 

 upon water, blow upon them with the mouth gently, till they are broken down, 

 and thus cover the film with any one colour, suppose green of the third order. If 

 the film is now placed vertically, the colouring matter will resume the form of bands, 

 as in Fig. 8, the colour produced by blowing, namely, the green of the third order, 

 taking its proper place on the film in a very broad band, having bands of inferior 

 orders above it, and very narrow bands of higher orders below it. 



The phenomena produced by this process are innumerable and highly beau- 

 tiful, varying with the order of the number of tints originally on the film, and 

 with the order of the tint produced by blowing, whether the blowing commences 

 before any bands appear, or after most of them have quitted the film. 



The special colour produced over the whole film by blowing is not composed 

 wholly of colouring matter of one order, though that matter predominates. When 

 examined by the microscope the film exhibits a most beautiful combination of 



