512 SIR DAVID BREWSTER ON THE FIGURES OF 



into a plane. When the diameter of the bubble is equal to that of the ring, the 

 ring will form its equator, and the bubble will be a perfect sphere passing into a 

 plane film when burst. If, when the bubble is a complete sphere, with its metallic 

 equator, we place another upon it, and alter its size by blowing and sucking 

 through the pipe that holds it, we shall observe the production of plane, convex, 

 and concave films, which has been already mentioned. When the bubble bursts, 

 a double hollow convex lens is often formed with particular solutions, though a 

 plane film is most frequently the result. 



When a bubble is placed upon the mouth of an open cylinder like the chimney 

 of a lamp, the glass being quite dry, it will deposit a film, which will immediately 

 move down the tube about an inch, and the bubble will burst, leaving another 

 film in its place. Both these films sometimes remain, the uppermost being con- 

 vex and the other concave. If we close the lower end of the tube by plunging it 

 in a tumbler of water, a bubble laid upon its mouth will deposit a plane film there. 

 If we now lift the tube slowly, the plane film will descend, becoming concave 

 before it bursts. By continuing to lift the tube the truncated bubble will grow 

 less and less, till it becomes a plane film about the eighth of an inch below the 

 rim of the tube. If we now depress the tube the film will gradually rise to the 

 primitive bubble. When the bubble is small it descends about one or two inches 

 as a convex film. If the diameter of the primitive bubble is a little less than the 

 diameter of the tube, it generally deposits itself within the tube in the form of a 

 semicylindrical film. 



In all conical tubes closed at one end, the film, 

 taken up by immersion, or deposited by a bubble of 

 considerable size, is always convex. The film is also 

 convex in closed cylindrical tubes if the mouth is 

 slightly widened as in test tubes. If the bubble de- 

 posited upon cylindrical tubes of this kind is very 

 small, but not less in diameter than their own, the 

 film is plane, and sometimes slightly concave, de- 

 scending a little in the tube. If the bubble is smaller 

 still, it forms itself into the semicylindrical film 

 already mentioned. 

 If we employ a truncated cone the phenomena and motion of the films are 

 very remarkable. When the cone ABDC is closed at CD, the other end, AB, 

 gives by immersion a plane film. When a bubble is laid upon AB it forms a 

 concave film. Upon admitting the air gradually at CD, the concave film descends, 

 expanding itself into a larger plane film which breaks at CD. The bubble then 

 follows it, leaving a plane film at AB. When the cone ABCD is closed at AB, it 

 gives, by immersion, a convex film, which by the gradual admission of the air at 

 AB descends as a plane film contracting itself to AB. 



