420 Prof. Gr. Quincke on the Edge-angle and 



Such a porous partition behaves similarly to a more or less 

 completely closed valve, and permits, according to its nature 

 or chemical properties, the liquid 2 to flow towards liquid 3, 

 or liquid 3 towards liquid 2. 



The experiments mentioned show that 



a (water, bladder) < a (alcohol, bladder) 



a (water, indiarubber) > a (alcohol, indiarubber). 



The surface-tension at the boundary of a liquid and a solid 

 body appears, like the surface-tension at the boundary of two 

 liquids, to be less in proportion as one liquid is the more able 

 to dissolve the other body. 



Besides, it is in most cases very difficult, in single experi- 

 ments, to distinguish the surface-diffusion, through the inter- 

 position of the solid partition from the free diffusion which 

 occurs between two liquids without the interposition of the 

 solid partition. That is also the reason why these processes 

 have hitherto been so little investigated. 



Since the friction of the liquid particles against one another 

 decreases with augmenting temperature, with an augmenting 

 temperature the diffusion must increase for equal differences 

 between the magnitudes a 12 and a 13 , where again the solid 

 is denoted by 1 and the diffusing liquids by 2 and 3. And 

 experience generally* agrees therewith. 



12. Imperceptibly thin films of liquid. Creeping of salts. — 

 Many times already have I drawn attention to the want of 

 harmony between theory and the observed facts, and at the 

 conclusion of § 9 have inferred therefrom that imperceptibly 

 thin films of a foreign substance upon the surface of the solid 

 body may be the cause of this accidental circumstance. 



On account of the numerous observations, and the accuracy 

 of the methods employed, it is specially remarkable that in 

 the measurements upon air-bubbles in aqueous saline solutions 

 an edge-angle of from 20° to 30° is generally found, and yet 

 by the direct method of reflexion a much smaller value is 

 found. 



It implies this, according to my thinking : — that clean 

 liquids spread instantly upon the clean surface of solid bodies, 

 just as clean fluids spread instantly upon the clean surface of 

 mercury ; and that if no spread occur, and the edge-angle 

 differ from 0°, the surface of the solid body, like that of the 

 mercury, is always coated with an excessively thin film of a 

 foreign substance which hinders the spread. 



This thin film (which itself escapes observation, because it 



* Briicke, Pogg. Ann. Win. p. 78 (1843). 



