VII] OF FLOATING DROPS 469 



than the sum of the other two ; otherwise the triangle is impossible. 

 In the case of a drop of olive-oil on a clean water-surface, the relative 

 magnitudes of the three tensions (at 15° C.) are nearly as follows: 



Water-air surface 59 



Oil-air „ 25 



Oil- water ,, 16 



No triangle having sides of these relative magnitudes is possible, 

 and no such drop can remain in existence*. 



(2) The three surfaces may be all ahke : as when two soap-bubbles 

 are joined together on either side of a partition-film. The three 

 tensions then are all co-equal, and the three angles are co-equal; 

 that is to say, when three similar liquid surfaces, or films, meet 

 together, they always do so at identical angles of 120°. Whether 

 our two conjoined soap-bubbles be equal or unequal, this is still 

 the invariable rule; because the specific tension of a particular 

 surface is independent of form or magnitude. 



(3) If all three surfaces be different, as when a fluid drop Ues 

 between water and air, the three surface- tensions will (in all likeli- 

 hood) be different, and the two surfaces of the drop will differ in 

 their amount of curvature. 



Fig, 153. 



(4) If two only of the surfaces be alike, then two of the angles 

 will be ahke and the other will be unlike; and this last will be the 

 difference between 360° and the sum of the other two. A particular 

 case is when a film is stretched between sohd and parallel walls, 

 like a soap-film within a cylindrical tube. Here, so long as no 

 external pressure is applied to either side, so long as both ends of 

 the tube are open or closed, the angles on either side of the film 

 will be equal, that is to say the film will set itself at right angles to 

 the sides. Many years ago Sachs laid it down as a principle, which 



* Nevertheless, if the water-surface be contaminated by ever so thin a film of 

 oil, the oil-drop may be made to float upon it. See Rayleigh on Foam, Collected 

 Works, m, p. 351. 



