146 Dr. A. Oberbeck on the Friction 



The first two scries show a small increase of the resistance 

 in the interval of an hour; in the last series it remains con- 

 stant. After a period of 24 hours the resistance has become 

 so great that it could not be determined by the method em- 

 ployed. After stirring, however, it falls back to only a little 

 less than the initial value. Although, therefore, it cannot be 

 disputed that longer contact with the air increases the resist- 

 ance, just as little can it be doubted that a special surface- 

 resistance is present immediately after the formation of the 

 free surface. Hence we must conclude either that a very 

 great surface-resistance belongs to the free water-surface, or 

 that a pure water-surface in contact with the air has no exist- 

 ence at all. We should then have, in the method of observa- 

 tion employed, a fine means of judging of the condition of a 

 water-surface. 



While in water and aqueous salt-solutions the increase of 

 resistance at the surface is very considerable, the decrease of 

 the same in the other liquids is proportionally small. It 

 amounts in 



Per cent: 



Alcohol to 11-9 



Alcohol solution of CuCl 2 . „ 8*6 



„ „ CaCl 2 . „ 5-4 



Oil of turpentine . . . . „ 12*6 



Sulphide of carbon . . . „ 26*3 



The behaviour of the last liquid is (as was previously found 

 to be the case) abnormal in consequence of the strong evapo- 

 ration ; so that a proportionally large diminution cannot be 

 surprising. In the rest of the liquids, moreover, the resistance 

 at the surface (contact with it by the upper edge of the plate) 

 is always still greater than on the motion of the plate at a 

 greater distance from the free surface. Hence I do not think 

 we are entitled to infer from these experiments a diminution 

 of the coefficient of friction at the surface. 



In solutions of water in alcohol we get: — 



At the sp. gr. 0*9708, increase 9 # 8 per cent. 

 „ „ 0-9274, decrease 10'3 „ 



From this it follows that even a trifling addition of alcohol 

 to water considerably lessens its characteristic property, while 

 on a further addition the mixture behaves nearly like pure 

 alcohol. This appears to me to stand in the closest connexion 

 with the rapid diminution of the cohesion of water by the 

 addition of alcohol. For it Quincke* gives the following 

 values : — 



* Pogg. Ann. clx. p. 368 (1877)- 



