SIR DAVID BREWSTER ON THE COLOURS OF THE SOAP-BUBBLE 



503 



A good plane film may be produced in a very singular way. If we deposit a 

 bubble on the mouth of a cylindrical or conical wine-glass, a little less in diameter 

 than that of the bubble, the bubble, according to its size, will leave on the glass 

 one-third, or one-fourth, or one-fifth of itself as a plane film, and will stand above 

 the film two-thirds, three-fourths, or four-fifths of a sphere. 



If we deposit the same bubble upon a cylinder of glass or metal, open at both 

 ends, it will deposit the lower portion of itself as a concave film upon the cylinder ; 

 and if we burst the bubble, the concave film will start into a plane one. Another 

 method of producing perfectly plane films has been already described. 



2. Convex Films. — A convex film is frequently produced upon a cylindrical 

 wine-glass, but always upon a conical one ; and we can easily convert a plane or 

 concave film, when formed upon a closed cylinder, into a convex one, by heating 

 the air within the cylinder. In some cases I have thus converted a plane film 

 into nearly a complete sphere. The same result may be obtained when the film 

 is at one end of a long open cylinder, by plunging the lower end in water. 



3. Concave Films. — Films of this kind are less easily obtained than those 

 which are plane and convex. They are often produced upon a cylindrical wine- 

 glass, as has been already stated ; and they may be 

 always produced by depositing a bubble upon the 

 end of an open cylinder. 



4. Plane, Convex, and Concave Films. — All these 

 films may be obtained in succession by the juxta- 

 position and partial union of two soap-bubbles of 

 the same or different sizes. 



Let A be a bubble, deposited upon a wine-glass 

 C or an open cylinder, and B another bubble laid 

 upon A, and kept there by the pipe PQ, supported 

 upon a stand QR. The two bubbles will be sepa- 

 rated by a film MN. If the bubble A is equal to B, 

 the film MN will be plane. If A is greater than B, 

 MN will be concave; and if A is less than B, MN 

 will be convex. If, when A is equal to B, we en- 

 large B by blowing through QP, MN becomes convex. 

 If we then diminish B, by sucking out the air at Q, 

 MN becomes concave. In all these three forms the 

 film MN is competely protected from air, and almost completely from extraneous 

 light* 



* After I had used this method of producing the three varieties of films, I found that M. 

 Plateau had long ago discovered the relation between the size of two united soap-bubbles and the 

 curvature of the film which separates them. 



VOL XXIV. PAET III. 6 U 



