AND FORMS OF LIQUID FILMS. 113 



• 

 These experiments succeed best with freshly-formed films, but they may be 



performed even after the colouring matter has formed on the film, though with 



less certainty. In re-forming the original film, as above described, the wine-glass 



may be held either horizontally, vertically, or even inverted entirely with the 



watch-glass held beneath it. 



(3.) If, after forming the concave film, as in the first experiment, we raise the 

 watch-glass from the wine-glass, maintaining the contact between the two only at 

 one point, and then cause the watch-glass to rotate by this point round the wine- 

 glass, the concave film will likewise move round the wine-glass ; and by continuing 

 the movement, the concave film may be gradually enlarged, till it passes into the 

 original single film, adherent to the wine-glass only as in the last experiment, the 

 curvature of its margin passing through the forms already observed. 



(4.) If, having formed the concave film as in the first experiment, we lift the 

 watch-glass perpendicularly from the margin of the wine-glass, the edges of the 

 concave film will remain adherent to the wine-glass and watch-glass respectively, 

 the film being stretched out, while the fluid between the glasses at their previous 

 line of contact will be drawn into a second film, which unites with the concave 

 film to form a cylindrical bag attached above to the watch-glass, and below to the 

 wine-glass.* By continuing to raise the watch-glass, this bag will sometimes 

 become detached from the watch-glass, and return into the form of the concave 

 film ; at other times it will leave the wine-glass entirely, and take the form of a 

 lens upon the watch-glass. Fig. 3 exhibits the cylindrical bag thus formed, still 

 attached to both wine-glass and watch-glass. 



As seen from the figure, the remainder of the wine-glass is now covered by a 

 separate film, upon which the cylindrical bag partly rests. This may be 

 regarded as that part of the original film which, in the first experiment, ran up 

 and attached itself to the concave surface of the watch-glass, and which, in 

 the process of perpendicularly lifting the watch-glass, has become restored to its 

 original position, and which, for the sake of distinctness, we may call the 

 complementary film. Whether the cylindrical bag will adhere to the watch- 

 glass or return into the concave film, seems to depend on the relation of the 

 radius of curvature of the concave film to that of the film covering the remainder 

 of the glass, and to that of the watch-glass itself. The smaller the concave 

 film, and the more convex the complementary film, the more readily will the bag- 

 leave the latter, and attach itself to the watch-glass in the form of a lens, while 

 the less concave the watch-glass, the more difficult it is to produce this result, 

 till, when we use a perfectly flat glass, it will rarely take place in any case. 

 When, however, the complementary film is either accidentally or purposely broken, 

 so that only the concave film remains, the latter will invariably pass into the form 



* The experiment succeeds best, by first raising the portion of the watch-glass most distant from 

 the concave film, and then lifting the whole watch-glass vertically. 



