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LXI. Experiments on the Viscosity of a Solution of Saponine. 

 By W. H. Stables and A. E. Wilson, Yorkshire College, 

 Leeds *. 



M PLATEAU has shown (Statique des Liquides, t. 2, 

 • ch. vii.) that a body placed in a liquid and wetted on 

 one side only, experiences in many cases a greater resistance 

 to its motion than if it were completely immersed. Some 

 controversy has arisen between Marangoni and himself as to 

 the cause of this phenomenon, which M. Plateau explains by 

 the assumption that the liquids in question possess a surface 

 viscosity greater than that of the interior. The liquid in 

 which the surface resistance is in most striking contrast to 

 that of the interior is a solution of saponine in water. 



Oberbeck (Wiedemann's Annalen, Bd. 11, S. 634) has 

 repeated and extended Plateau's experiments, using an oscil- 

 lating disk instead of a magnetic needle. He made no 

 observations upon saponine solution. The object, therefore, 

 of the following investigation is to study the movements of 

 a disk when oscillating in or near the surface of a solution of 

 this substance. 



Oberbeck found that the resistance of a water-surface 

 increased largely with exposure to the air ; but he also proved 

 that even with fresh distilled water the resistance is con- 

 siderable. As he points out, we are therefore led to one of 

 two conclusions — namely, that either water has a surface 

 viscosity different from that of the interior, or else that a 

 pure water surface cannot be obtained. 



The apparatus used in our experiments was similar to that 

 employed by Grotrian (Pogg. Ann. Bd. 157, S. 237). 



It is fully described by him ; and it is there- Fig. I. 

 fore unnecessary to figure here the connexions 

 of the different parts. The diagrammatic re- 

 presentation in fig. I. may, however, conduce 

 to clearness. 



It consists of a circular disk A of nickel- 

 plated brass 76*25 millim. in diameter and 2 

 millim. thick. In the centre is screwed a 

 brass rod, to which a concave mirror B and a 

 small iron bar C (used to set the apparatus 

 in motion by means of a magnet) are attached. 

 A wire (119*8 centim. long), employed to put 

 the apparatus in motion by torsion, is firmly 

 gripped between two small plates at D. — 



* Communicated by the Physical Society, having been read at the 

 Meeting held on April 14, 1883." 



