65 i SIR DAVID BREWSTER ON THE MOTIONS AND COLOURS UPON 



The currents which are produced upon films of the alcohol of commerce 

 appear also upon those of absolute alcohol ; but though the films are smaller and 

 less persistent, the currents are more active and varied in their movements. 

 Similar currents are produced upon films of various solutions containing alcohol, 

 of alcoholic solutions containing water and sugar, and of a large number of 

 volatile and fixed oils, which, through the kindness of Dr Playfair and Dr 

 Christison, I have been enabled to examine. They appear also, but with less 

 activity, upon films of a solution of New Zealand gum in oil of laurel— the re- 

 markable fluid by which the late Mr Delarue produced the brilliantly coloured 

 papers which were shown at the Great Exhibition of 1851. 



In all these experiments the principal current and its different ramifications 

 are perfectly colourless, and consequently exceed the thickness in Newton's scale 

 at which the colours of thin plates make their appearance. In good and per- 

 sistent films slight colours appear between the secondary currents ; but these, as 

 we shall presently see, are the complementary colours of those seen by reflection. 



The various phenomena which I have described may be seen in a magnified 

 form, by placing the films in a beam of divergent light, and they might be ex- 

 hibited to an audience by means of the magic lantern. 



If we now examine the surface of the alcoholic film by reflected light, we 

 shall observe a series of phenomena of a very different kind. The principal 

 current and its branches will be seen almost as distinctly as by transmitted light ; 

 but they are accompanied with, or rather they produce, systems of coloured rings 

 of great beauty, shifting their place on the film, expanding and contracting 

 quickly, and rapidly changing their form and their colours. Each pair of 

 systems has on one or both sides a secondary current which stops or disturbs the 

 rotatory motion, which would be communicated to two systems by the action of 

 a single current. 



When the film is first formed, especially if it is a very small one, there is only 

 one system which is maintained by the colourless fluid issuing from the margin, 

 sometimes in closely packed bands, of very high orders of colours. The lowest 

 colour is always in the centre of the system, but the central tint is never lower 

 than the white of the first order. When the tint occurs which Newton calls the 

 beginning of black, the film always bursts. 



This single system of rings is finely seen in films of very old balsam of 

 copaiba which I obtained from Dr Christison's Museum. The film was wholly 

 occupied by a circular system contracting and expanding quickly, changing its 

 central tint, becoming elliptical, and even of a crescent form, when pushed onward 

 by the thicker fluid from the margin of the film. These movements were kept 

 up for more than an hour without any rotatory motion, and had not the film 

 burst from an accident, they might have continued much longer. 



After the single system of rings has appeared upon a film of alcohol, the film 



