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



quantity of water, the film which it gives is much smoother. Two or three 

 serrated bands appear at its apex when it is placed vertically, and the streams 

 of colour flow quickly from the circumference. In one of 

 these films the singular black figure shown in the annexed 

 diagram was produced. The film immediately burst before 

 exhibiting any colours. 



When the film from this solution is first made, it is almost 

 always perfectly colourless, and remains so for a short time ; 

 but if we blow on it in this state, through a narrow tube, 

 along a diameter or otherwise, the most brilliant systems 

 of revolving rings are produced, as in Figs. 9 and 10, a result obtained from the 

 soap-film only when the colours had previously existed upon it. The first rings 

 that are seen in this experiment are colourless. The highest order of colours then 

 appear in the centre of the system which is most influenced by the blast, and 

 this is succeeded by the next lower order, till the centre becomes black, and the 

 film bursts. When the blowing ceases, the colours often wholly disappear, the 

 colouring matter being restored by a gradual rise of tint to its colourless state, as 

 it was in the newly-made film. If we place the film vertically, after it has been 

 blown upon, an approximation to mottled bands appears, the greater part of the 

 film being colourless. If we blow upon the newly-made film with the mouth, 

 instead of a tube, no colours are produced. 



V. On the Origin and Development of the Colours on the Soap-Bubble. 



It is impossible to witness the simplest of the preceding experiments without 

 being convinced that the common theory of the colours of the soap-bubble is incor- 

 rect, and that they are not produced by different thicknesses of the elastic film which 

 composes the bubble. A colouring matter of a very peculiar kind floats upon the 

 bubble, or upon the film, as oil does upon water, or as the oil of laurel varnish of the 

 late Mr Delarue does, in producing those magnificent colours which he succeeded 

 in transferring to paper. In these cases, however, the colours are caused by the 

 mere expansion of the oil or varnish into thin films, producing what Newton calls 

 the colours of thin plates ; but in the case of the soap-bubble the colours are 

 formed by minute molecules, either of the soapy solution itself, or, what is more 

 probable, by some of its ingredients or elements expressed or secreted from it 

 only when in the state of a film, forming, under the influence of gravity and their 

 mutual attraction, the different orders of colours we have described. 



This curious process, of which there is no other example in the production of 

 colours, may be traced experimentally with the microscope from the formation of 

 the elastic film till its explosion ; but by what process the colourless secretion 

 parts with the colorific molecules which compose it, and by what laws and affinities 

 these molecules take their place in the different orders of colours, enter into those 



