7(38 .Mr. H. D. Arnold on Stokes's Law for 



always remain irregularities of molecular size which might 

 still be efficient in preventing slip, it does not seem un- 

 reasonable to compare these results with those obtained with 

 somewhat more highly polished surfaces. A further con- 

 firmation of the high value of /3 was found in experiments 

 with the same spheres in castor oil of viscosity 7*0. The 

 lack of evidence of slip in this case implies /3 greater than 

 50000. It should be remarked that as far as these or any 

 previous experiments are concerned, j3 may still have a finite 

 value, and indeed according to the kinetic theory of fluids it 

 is inconceivable that it should be infinite. The experiments 

 simply fix a limit which the- value must always exceed, and 

 it is quite probable that for sufficiently minute spheres this 

 factor would become of importance. It is interesting to 

 note the resemblance between the factor here used to correct 

 for surface slip and that arrived at by Cunningham* from a 

 quite different standpoint. 



In an attempt to investigate the slip at other surfaces, 

 spheres of various liquids were used in liquids of nearly the 



I same densities. But here another disturbing factor enters, 



as one quickly realizes when he watches the rise of a drop of 

 alcohol through olive oil. The outer layer of the drop is 

 dragged away by the oil, and eventually breaks up into a 

 chain of minute droplets. This wearing away of the surface 

 Avould give the same effect as surface slip, but seems to be 

 more than offset by the accompanying distortion of shape. 

 The net result is a great irregularity in the results with 

 different spheres, and a computed value of /uu usually somewhat 

 too high. 



Some preliminary experiments on the simultaneous motion 

 of air bubbles and mercury spheres in the same tube were 

 made in the hope that by thus entirely obviating the tempe- 

 rature factor, the method might be made sufficiently sensitive 

 to detect much smaller slip effects. It at once appeared 

 that the apparent slip at the surface of the air bubble was 

 infinite in some liquids and zero in others. This wholly un- 

 expected phenomenon gives us a method of verifying the 

 correctness of the theoretical slip correction factor. 



For this purpose a tube 2 "4 cm. in internal diameter, and 

 27 cm. in length, was sealed to a narrow tube about 10 cm. 

 in length and the outlet fitted with a glass stopper having a 

 fine capillary opening. This opening was sufficiently small 

 to retain the liquid and assured atmospheric pressure through- 

 out the tube. The narrow stem was for the purpose of 

 retarding the motion of the bubble so that it might not be 

 . ".*. Loc, cit. 



