432 On the Spread of Liquids on Solid Bodies. 



5. The edge-angle may be determined indirectly by de- 

 ductive calculation from the measurement of the form of flat 

 drops and bubbles, or directly by reflection of light. 



6. The edge-angle of the free surface of various liquids, 

 such as water, alcohol, etc., and aqueous or alcoholic solutions 

 of salts, upon clean surfaces of glass, crystals, or metals appears 

 to be 0°. The liquids spread upon the clean solid surfaces. 



7. If the edge-angle has, as is usual, greater values, then 

 the solid surface has been covered with an (imperceptibly) 

 thin film of a foreign substance, with the thickness of which 

 the edge-angle alters. 



8. The thickness of this thin film, however, cannot exceed a 

 certain maximum value D, which is as great as, or greater 

 than, the radius of the sphere of sensible action of the 

 molecular forces. 



9. This thin film adherent to the surface of the solid body 

 may consist of solid, liquid, or gaseous matter. 



10. It may also itself consist of the liquid deposited, and 

 may be demonstrated, besides by the edge-angle, by the so- 

 called " creeping " of salts, or by electric conduction on the 

 surface of the solid body, and in single cases also by the in- 

 terference-tints of the light reflected by it. 



11. The imperceptibly thin films of the same liquid possess 

 different properties according to the duration and the nature 

 of their origin, or according to the nature of the solid body to' 

 which they adhere. 



12. Rapidly formed drops of water spread more readily 

 upon freshly cleansed surfaces of glass than those formed 

 slowly. 



13. These imperceptibly thin films of foreign matter appear 

 to be the cause of the deviations between theory and experi- 

 ence in the determination of the surface-tension at the common 

 boundary of liquids and solid bodies. 



14. When the edge-angle is 0°, or has impossible values, 

 there results a spread of the liquid upon the surface of the 

 solid body. 



15. In the case of liquids which are miscible in all propor- 

 tions, the liquid with the less surface-tension a 12 drives away 

 that with the greater surface-tension a 12 . This surface- 

 tension and the driving-away which may occur, change, how- 

 ever, with the nature of the solid substance. This completes 

 Bnicke's theory of the surface-diffusion along a solid surface. 



16. The presence of other fluids, and particularly of air, 

 may essentially modify the spread of a liquid upon a solid 

 surface. 



17. The dependence of the edge-angle upon the thickness 



