FILMS OF ALCOHOL AND VOLATILE OILS AND OTHER FLUIDS. 655 



gradually increases in breadth from evaporation, and the principal current throws 

 out a secondary current, as in Fig. 3, which throws the colouring matter into two 

 systems of rings that seem to move, the one from right to left, and the other from 

 left to right, like those produced on the soap film by a current of air. 



Another secondary current gives rise to three systems of rings, as in Fig. 4, and 

 several such currents to several systems, as in Fig. 5, in which the motions are so 

 rapid that it is difficult to follow them. 



In this condition of the film the principal current becomes flat, and itself 

 becomes divided, as it were, into narrow bands, and the various systems of rings 

 unite into one singular system, rudely represented in Fig. 6 ; the tints beyond the 

 salient points being at the margin of the film, the white of the first order gradually 

 rising to the higher tints of Newton's scale. The contracting and expanding 

 motions of this system become slower and slower, and the film generally bursts, 

 scattering its fluid round the aperture upon which it was formed. 



In some cases, which are very rare, the whole system of rings disappears with 

 the principal current — the film becomes perfectly quiet and colourless, and, as in 

 the soap film, the colouring matter, in the form of bands and tadpoles, comes 

 upon it from its margin, till the black of the first order covers the film, and causes 

 it to burst. This is a very instructive result. 



Owing to the irregular movements of the secondary currents, it is difficult to 

 observe the direction of the motion of the systems of rings in Figs. 3, 4, and 5 ; 

 but they certainly revolve in opposite directions, like those produced by an arti- 

 ficial current in the soap film. When there is only one system of rings it has 

 not a rotatory movement, because there is no decided current to put the colouring 

 matter in motion ; but when there is a principal current, with several secondary 

 ones, as in Figs. 4 and 5, the secondary currents prevent, by their opposite actions, 

 the intermediate rings from revolving. 



The systems of rings which I have described are seen with various modifica- 

 tions, in about 70 or 80 volatile and fixed oils, and other liquids, which I have 

 had an opportunity of examining. In most of them the motions of the cur- 

 rents and rings are very rapid. In some a film cannot be obtained, as in the 

 ground nut-oil, the oil from the Sesamum orientate, and the purified oil of bitter 

 almonds. In others the film, though it exhibits the play of colours, does not 

 exceed the 50th or 60th of an inch in diameter. In Elaine, from olive-oil, 

 and in the oil from the Jatropha eurcus, a very small film is obtained, which 

 instantly becomes black, and disappears. In Mr Delakue's solution of New Zealand 

 gum in oil of laurel, the motions of the rings are very languid, but the film is 

 very persistent ; and, what is interesting, I observed a number of black particles 

 pass across the rings, and break their outlines, showing that the colouring matter 

 floated upon the elastic film, as in the soap-bubble. The same effect was some- 

 times produced by blowing upon the film. 



