148 



♦ KNOAVLEDGE 



[Maech 1, 1886. 



this way : and the broad, dark sea-floors attest the energy 

 with which for awhile such work weut on, while the 

 ghosts of craters buried beneath the sands of those lunar 

 seas show that ouce, if no more, sea replaced land even in 

 the moon. 



SOAP-BUBBLES AND FILMS.* 



Br T. O'CoNuE Sloanp, Ph.D. 



3E true nature of a liquid 61m is compar- 

 able to that of a perfectly elastic and 

 tightly-stretched membrane. All liquids 

 are bounded and enclosed by such a 

 membrane, compose:! of the substance of 

 the liquid itself. The phenomena of 

 films, under the form of soap-bubbles, 

 liave been known for manj" generations. 

 Thej- were seriously studied by Sir Isaac Newton, and 

 later by the scientist Dr. Plateau, of Belgium, a curioTis 

 study for one, like the latter, afflicted with total blind- 

 ness. . 



If a ring one or two inches in diameter, and provided 

 with a handle, is dipped into a solution adapted for 

 forming films, and is withdrawn, it will be found to be 

 filled with a beautiful film, straight and firm, reminding 



Fig. 1. 



Us of tlie wing of a dragon-fly (Fig. 1). If we blow 

 against it, it will be driven out into a purse-like .shape of 

 very characteristic outline (see dotted line). If it be 

 held between the mouth and a candle, it will screen the 

 latter from strong blowing until it breaks, when the 

 candle will be extinguished. 



Fig. ■>. 



By particular management a hole of any desired size 

 can be made in the side of a soap bubble. This is done 

 by tying a small loop, less than the third of an inch, in 

 the end of a silk thread, moistening it thoroughly with 

 the solution, and hanging it over the bowl of a pipe 



* From a lecture on "The Physics of Tenuity," given in full, 

 with many additional illustrations, e.xperimeuts, and formulas, 

 n Supplement to Scientifc American. 



just before blowing a bubble. As the bubble is blown, 

 the end of the thread and the loop will adhere to it. 

 Then by touching the film within the loop, either with 

 a hot wire or with a piece of blotting-paper, the film 

 will break inside of the loop, which will fly open to its 

 widest extent (Fig. 2). The bubble will immediatelj' col- 

 lapse, or by vigorous blowing may just be kept inflated. 

 The bla.st from the hole is sometimes enough to extinguish 

 a candle. 



This shows that the film is elastic. To measure 

 directly the tension exerted by an inflated bubble, a glass 

 tube bent at a right angle may be attached to the end of a 

 pipe-stem. After blowing a bubble, the end of the gl;:ss 



Fig. 3. 



tube may be dipped into water, when the depression will 

 show the pressure (Fig. 3). It will be but a small 

 fraction of an inch. 



To measure the tension of the film per unit; of surface, 

 a little frame with grooved sides is employed. In the 

 grooves a wire carrying a little scale pan slides freely 



Fig. i. 



up and down (Fig. i). The wire is pushed home to the 

 top of the frame and some of the solution introduced, 

 either by dipping the top or by painting it in with a 

 brush. Then, by adding weights, the film can be pulled 

 down like a delicate curtain until the limit is reached, 

 and it breaks. 



By mounting a ring as a pendulum and filling it ^-vith 

 a film (Fig. .5) the retardation the latter exercises on its 

 swing is quite striking. 



Four of the rings may be mounted as a windmill 

 (Fig. 6), and be made to turn sever.il times by the breath 

 until their perishable sails break one by oue. 



If a thread, well moistened with the solution, is laid 



