112 SIR DAVID BREWSTER ON THE MOTION, EQUILIBRIUM, 



position, this drop runs down the inner surface of the glass till, reaching its 

 lowest point, it brings the film into close contact with the watch-glass ; the film 

 now attaches itself to and spreads itself over the surface of the latter in a similar 

 way to what takes place when a bubble blown from a pipe is brought into contact 

 with any smooth surface, whilst the original system of equilibrium being now 

 disturbed, the upper part of the film is put in motion, and a new system of equi- 

 librium is formed. 



In making this experiment, the rise of the film upon the surface of the watch- 

 glass is generally so rapid, that it is difficult to watch its progress, but by careful 

 inspection it may be observed to commence first at a single point; the edge of 

 attachment of the film to the watch-glass then becomes first elliptical, then cir- 

 cularly concave, then a straight line, then convex, as seen in fig. 2, where the 

 lines represent the edge of the film on the watch-glass in its different stages of 

 progress. The changes which take place in the curvature of the concave spherical 

 film within the wine-glass during these stages are also very curious. The concave 

 film may sometimes be produced without inverting the glass, by holding the 

 watch-glass firmly in position with the thumb, and briskly shaking the glass. 

 The drop of liquid is thus thrown upon the margin of the film. 



(2.) The concave spherical film being thus formed, we can reduce matters to 

 their original state {i.e. reproduce the single original film on the margin of the wine- 

 glass) by slowly and carefully removing the watch-glass. The experiment is best 

 made by lifting the watch-glass from its connection with the wine-glass at its 

 upper margin, keeping the two in close contact at their lower margin. The upper 

 edge of the concave film will thus again rise in the wine-glass, while its lower 

 edge will descend along the watch-glass ; and when it has reached the point where 

 the two glasses are in contact, the watch-glass may be removed, and the original 

 film left on the wine-glass. The edge of the film, as it descends the watch-glass, 

 exhibits the same series of curves as it did in ascending in the last experiment ; 

 but, of course, in a reversed order. The nature of these curves may be much 

 more satisfactorily observed in this experiment than in the last, as their progress 

 is much less rapid. Before entirely removing the watch-glass, we can cause the 

 film to ascend or descend at pleasure, with any degree of rapidity, by approxi- 

 mating or separating the two glasses. 



In making the first experiment, the drop of fluid which causes the film to 

 attach itself to the wine-glass generally entirely escapes; consequently, if we 

 repeat this experiment upon the film as re-formed in the second experiment, it 

 will seldom succeed, there being no superfluous fluid to produce the necessary 

 contact. If, however, we place a drop of fluid, either on the surface of the 

 reformed film or on the inner surface of the watch-glass, the experiment will 

 succeed, and may be repeated indefinitely, till the film bursts — a fact which 

 clearly proves the drop of fluid to be the agent in producing the phenomenon. 



