1885.] Professor A. W. BiicJcer on Liquid Films. 243 



WEEKLY EVENING MEETING, 



Friday, March 20, 1885. 



Sir Fredeuick Bramwell, F.R.S. Manager and Vice-President, 

 in the Chair. 



Professor A. W. Piijcker, M.A. F.R.S. 



Liquid Films. 



The molecules in the interior of a liquid are surrounded on all sides 

 by others which they attract, and by which they are themselves 

 attracted, while those on the surface have neighbours on one side 

 only. In consequence of this difference in their surroundinorg there 

 is in all probability a difference in the grouping of the interior and 

 exterior molecules which is attended by corresponding variations in 

 the physical properties of the liquid of which they are constituent 

 parts. Thus it was shown by M. Plateau that the viscosity of the 

 surface of a liquid is in general different from that of its interior. 

 The most striking example of this phenomenon is afforded by a 

 solution of sajjonine. Two per cent, of this substance dissolved in 

 water does not effect any marked change in the properties of the 

 great mass of the liquid, but produces a most remarkable increase in 

 the surface viscosity, so that forces which suffice to create rapid 

 motion in bodies which are completely immersed, fail to produce any 

 ajjpreciable movement if they lie in the exterior sui-face. The first 

 attem23t to obtain a numerical estimate of the difference of the resist- 

 ances experienced by a body oscillating in turn in the interior and in 

 the surface of the liquid was made about two years ago by Messrs. 

 Stables and Wilson, students in the Yorkshire College. In the case 

 of a horizontal disc suspended in water, the logarithmic decrement 

 diminishes to about one-half as the surface is approached. In a 

 saponine solution, on the other hand, it is 125 times greater in the 

 sui'face than in the interior, and about 38 times greater in the surface 

 than at a depth of 0*1 mm. below it. Even in the latter case the 

 greater part of the resistance is due, not to the friction between the 

 disc and the liquid, but to that experienced by the supporting rod in 

 the surface, so that in all jDrobability the surface viscosity is more 

 than 600 times greater than that of the mass of the liquid. 



The immense change in the resistance which takes place when the 

 disc is immersed to a depth of O'l mm. only confirms the general 

 opinion that any peculiarity of grouping or arrangement due to 

 proximity to the surface extends to a very small depth. A liquid 

 must thus be conceived as surrounded bv a very thin layer or skin, 



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