336 



Prof. G. Quincke on the Edge-angle and 



Upon the same surfaces of glass and silver, which were 

 cleansed with alcohol, water, and a clean linen cloth, I found 

 by both methods the following values of the edge-angle for 

 mixtures of alcohol and water of various specific gravities: — 



Table VII. 



Alcohol of 

 specific 

 gravity. 



With air-bubbles. 



By reflexion. 



Glass. 



Silver. 



Glass. 



Silver. j 



0-9973 

 09852 

 0-9200 



30 53 

 26 23 

 16 21 



43 31 

 62 18 

 20 21 



20 34 

 15 36 



14 28 



72 14 

 62 35 

 25 42 



With the exception of the one determination in the case of 

 very dilute alcohol and silver, where the fluid surface of the 

 air-bubble was very difficultly movable, and a casual impurity 

 may have produced a difference, the results of both methods 

 of observation agree as far as can in general be expected in 

 these investigations. 



9. The magnitude of the surface-tension a 12 at the boundary 

 of a solid and of a liquid may be determined to within an ad- 

 ditive constant so soon as the tension of the free surface, and 

 the edge-angle for various liquids upon the same solid (for ex- 

 ample glass), are known. 



From equation (5), § 1, we have for the fluids 2 and 3 : — 



ai2= a i — a 2 C0S #2? G> A ) 



a 13 = a l _ «3 COS ^3 ; 



or, by subtraction, 



a i2~~ a i 3 = a 3 cos S — a 2 cos 2 . . . (5 c) 



Call h the mean height of ascent in capillary tubes of dia- 

 meter 2r for a liquid of specific gravity a ; then, from equa- 

 tion (9), 



a 2 cos 6 2 = (ct)=rh-, 



whence follows at once the capillary constant (a) of the free 

 surface of the liquid concerned, as it used formerly to be cal- 

 culated from the height of capillary ascent in glass tubes under 

 the assumption that the edge-angle was zero. 



Moreover the value of a 2 cos ^2 ma y a ls° be calculated from 

 observations on flat air-bubbles beneath a level plate of glass. 



Comparing a 12 for various liquids with 



(5b) 



« 13 = iB 



