191(5] on Problems in Capillarity TDl 



so that the diffusion Avoukl diminish fairly geometrically. In the 

 example given above the relative i-ate of tlie double film was j\th 

 that of the single film, while the addition of a third film again 

 reduced this by gVtli- 



If these bubbles were actual holes, then the gas transit would be 

 that of simple transpiration. This would give a curve of an entirely 

 different character to those shown in Fig. 2. The plots of log. 2) and t 

 would no longer give a straight line, as the rate of gas transit is then 

 proportional to the square root of the pressure. This was actually 

 tried on a capillary tube, when the appropriate curve of diminution 

 of pressure with time was obtained. 



The bursting pressure of a collodion black film of double thick- 

 ness was found by connecting the open end of the polished glass tube 

 on which it w^as mounted, as described above, to a mercury manometer. 

 Air pressure was gently introduced, and the height of the mercury 

 pressure required to burst the film observed. As an example, a 

 film of double thickness, o^ mm. in diam., burst at o cm. pressure 

 of mercury, corresponding to a load of 40 grms. weight per sq. cm. 



Soap Films ix Exhausted Vessels. 



Films of soap solution can readily be obtained of the same order 

 of thickness as those from collodion. AVhen produced in exhausted 

 vessels they are freed from many disturbing agencies. The deleterious 

 effects of suspended matter and other air impurities are avoided ; 

 convection currents and air friction disappear : and the motions in 

 the film are rendered more definite. The solution being free from 

 dissolved gases, the absence of oxygen and carbonic acid stabilises 

 the soap solution, Avhich, however, may in time be damaged by 

 attack on the glass. Any froth that may be produced rapidly 

 subsides, because the bubbles are formed only of water vapour, 

 which condenses quickly owing to the higher pressure inside the 

 bubbles. 



The three methods of film production in exhausted vessels most 

 frequently used are : — 



(«) Production of bubbles of w^ater vapour from the slightly 

 warmed soap solution, or by violently shaking the liquid until it 

 becomes a mass of froth and allowing to stand. 



(Jb) " Throwing " the film by a smooth and rapid circular swish 

 of the soap solution over and round a constricted neck : or across a 

 cylindrical vessel. 



(c) Immersing, or otherwise covering, a wire or glass framework, 

 wliich is thus left holding a film when the soap solution is carefully 

 withdrawn from it. 



The first method is applicable to an ordinary distilling flask, or 

 plain exhausted tube, such as are shown in Fig. 4. The flask has a 



