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



The glass plate was rubbed with olive-oil and a clean linen 

 cloth till all visible oil was removed ; the edge-angle of the 

 same plate for water was now 



51° 55'. 



The drop of water was then after some minutes poured off, the 

 last traces evaporated, and after deposition of a new drop with 

 a less surface of contact the edge-angle was 



42° 10'; 



and on a repetition of the same operation, 



31° 53'. 



The glass surface behaved, therefore, like a quicksilver sur- 

 face, upon which, as I have formerly shown in detail*, water 

 has a greater or a smaller edge-angle, after it has been coated 

 with film of foreign fluid more or less thick, provided the 

 thickness of the film is less than 2Z, or less than double the 

 distance at which the molecular forces of capillarity are still 

 operative (see § 12). 



In the researches here described, a portion of the film of oil 

 with which the glass plate was coated dissolved in the water ; 

 the thickness of the film became thereby less, and a freshly 

 deposited drop of water showed a less edge-angle. 



Between the surfaces of the solid glass and the fluid mer- 

 cury there obtains, however, the essential difference that the 

 bounding surface common to water and mercury is easily dis- 

 placeable, but that common to water and glass is very diffi- 

 cultly movable. 



In the case of water and mercury, when the water is removed 

 in portions, the normal edge-angle is forthwith restored ; in 

 the case of water and a solid, such as glass, the edge-angle 

 becomes smaller. 



In the earlier researches with surfaces of mercury f , it was 

 possible, by applying very small quantities of oil to the free 

 surface of the mercury or of the water, to diminish the tension 

 of these surfaces and to give a lesser or a greater diameter to 

 the drop of water, and a greater or a lesser value to its edge- 

 angle. 



This experiment does not succeed in the case of drops of 

 water upon glass or any other solid body. 



If the free surface of a drop of water freshly deposited upon 

 a glass plate be touched with a glass thread moistened with oil, 

 a portion of the oil spreads upon the free surface, but the edge- 



* Pogg. Ann. cxxxix. pp. 66 & 72 (1870); and Phil, Mag. [IV.] 

 vol. xli. pp. 374 & 460 (1871), 



t Pogg. Ann, cxxxix. p, 67 ; Phil. Mag. [IV. J vol, xli. loc, eit, 



