( 653 ) 



XLIIL — On the Motions and Colours upon Films of Alcohol and Volatile Oils and 

 other Fluids. By Sir David Brewster, K.H., F.K.S. (Plate XXXIX.) 



(Read 4th March 1867.) 



In a paper " On the Phenomena of Thin Plates of Solid and Fluid Substances 

 exposed to Polarised Light," published in the "Philosophical Transactions" for 

 1841,* I had occasion to notice certain motions and colours which I had observed 

 upon films of some of the volatile oils ; but as they were unconnected with the 

 subject I was then investigating, I made no attempt to discover their nature and 

 origin. Their apparent similarity, however, to the molecular movements and 

 colours upon the soap-bubble, induced me to resume the subject, and to examine 

 them as exhibited upon films of various evaporable liquids, stretched over aper- 

 tures differing in size, form, and substance. 



If we place a drop of alcohol upon an aperture C, C, C", C ", Fig. 1 , held hori- 

 zontally, about the fifth of an inch or less in diameter, a concave lens will be formed 

 upon it. As the alcohol evaporates, a very small plane film will appear in the 

 centre, and will gradually increase in size till it fills nearly the whole aperture. If 

 we hold the film in a vertical position, and examine it by transmitted light, we shall 

 see a current of fluid, C, rising from the circumference of the film, moving rapidly 

 from one part of the circumference to another, occasionally taking a horizontal posi- 

 tion, and sometimes descending from the apex and sides of the film, as shown at 

 C, C", C". 



The current C is sometimes broad and flat, and separates into two currents, 

 M, N, Fig. 2, which dance, as it were, opposite each other, assume the form of the 

 letter S, and turn heels over head when they quit the circumference of the film. 



These apparent currents generally throw out secondary currents, as in Figs. 

 3, 4, and 5, and the whole of them continue in rapid motion, exactly like a 

 transparent insect struggling to escape. 



When an excess of fluid is placed in the aperture held vertically, it occupies 

 the lower part of it, and the film, no longer circular, appears in the upper part ; 

 but notwithstanding this change in its form and condition, the currents upon it 

 present the same phenomena. 



If the film is formed upon apertures of an irregular shape, it has, of course, the 

 same shape, but the form and motions of the currents are not changed. 



In some cases small currents issue from a part of the circumference of the 

 films opposite the principal current ; and in other cases small globules drop from 

 the extremity of the principal one. 



* Phil. Trans. 1841, p. 43. 

 VOL XXIV. PART III. 8 P 



