﻿of the common Surface of two Liquids. 465 



Instead of forming a lens-shaped drop, the water may also 

 cover the mercury as a thin layer with a circular opening. The 

 angle of the water with the surface of the mercury is the same 

 in both cases. 



28. If a drop of liquid 1 be placed on a liquid 2 so that 



*1 > a 2» «l2< a l"" tf 2* 



then liquid 2 will spread over the whole drop of liquid 1, and 

 this will assume a lens-shaped form. 



If a drop of water be placed on olive-oil, then its upper surface 

 will be coated with a thin layer of oil, so that a small drop of 

 oil, as Ludtge* has shown, will spread on it as on a free surface 

 of oil. The greatest diameter of the drop of water considerably 

 exceeds the diameter of the circular line of intersection of the 

 upper surface of the lens and of the free surface of the oil, which 

 in this place is conically depressed. The drop has something 

 of the form shown in fig. 3. This is in perfect agreement with 



Fig. 3. 



CC 2 C6S 



theory, since on an element of the circular line of intersection 

 the following tensions act (according to Table X. § 10), 



a 2 = 3'76, a 12 =2*10 milligrms., 



« 3 =3-76 + 2'10=5-86 



in which « 3 denotes the tension of the surface of the oil-layer on 

 the upper surface of the lens. 



The diameter of the circular line of intersection increases or de- 

 creases when a 3 is diminished by placing oil of turpentine on the 

 upper surface of the lens, or when a 2 is diminished by placing 

 the same liquid on the free surface of the oil. The increase or 

 decrease, however, is far from being so striking as in the analo- 

 gous phenomena with lens-shaped drops of water on mercury. 



29. Just as the vapours of foreign liquids contained in the 

 atmosphere condense and spread over free surfaces of mercury, 

 so also do they on the free surfaces of other liquids, e. g. water. 

 If these impurities are not dissolved by the condensing liquid 

 1 with great facility, they form a thin coating, which may exer- 



Pogg. Ann. vol. cxxxvii. p. 372. 



