W. p. Cooper and W. H. Nuttall 225 



peculiar superficial viscosity or rigidity, first observed by Plateau 

 (1873, pp. 261-96). In tlie case of saponin, tliis surface viscosity or 

 rigidity may be readily demonstrated. If a bubble is blown with a 1 % 

 solution of saponin, the bubble film will be found to be imperfectly 

 elastic, and quite difl^erent from a soap film. If air is withdrawn while 

 the orifice of the pipe is held in a vertical plane, so as to disturb the 

 symmetry of the bubble, the shrinking film assumes a crinkled appear- 

 ance, until finally there results a vertical semi-solid cylinder with almost 

 opacjue walls. 



Vermorel and Dantony (1912, pp. 1300-1) were the first to point 

 out that a hquid, exhibiting surface viscosity to a marked degree, had 

 in consequence a high wetting power. Solutions of saponin have 

 marked wetting properties, although the surface tension of such solutions 

 is comparatively high. Thus, a 1 % solution of saponin is capable of 

 wetting a glass plate, coated with paraffin wax, although a 5 % solution 

 of soap fails to do so. The wetting power of solutions of saponin, 

 gelatine, etc., seems to depend largely upon their capacity to form 

 li(|uid planes, the high superficial viscosity of which prevents rupture 

 and running together to form drops. 



The phenomenon of surface concentration is, however, of interest 

 to us from another point of view. It has long been observed that the 

 apparent surface tension (static) of soap solutions is practically identical 

 for all concentrations between 10 % and 0-1 % (Marangoni, 1871, 

 p. 342, footnote). More recent investigations have, however, shown 

 that this statement is only true, if the surface tension determination has 

 been carried out on an old surface. Rayleigh (1890, p. 285) succeeded 

 in showing that, if the surface tension of a 0-25 % solution of sodium 

 oleate is measured within 1/400 of a second after the formation of the 

 surface, the surface tension (dynamic) approximates closely to that of 

 water. This initial tension, however, quickly falls until the ordinary 

 value for the surface tension is reached. Rayleigh's results have been 

 repeatedly confirmed by other workers, and a distinction is now drawn 

 between what are known as the static and dynamic surface tensions. 

 The static tension is the value obtained, when the determination is 

 carried out on an old surface, as with the capillary rise and stalagmometer 

 methods. The term 'surface tension' as generally used in textbooks 

 refers to static surface tension. The dynamic surface tension on the 

 other hand is the value obtained with determinations on perfectly 

 fresh surfaces, as in the methods of surface tension determination by 

 means of jets, surface ripples, etc. The dynamic tension is always ill- 



