N 



t'APILLARY ACTION. 



motive fcwe M from tlie acid to the mercury the surface-tension increases; if 

 it H from the roereoiy to the acid, it diminishes. Faraday observed that a 

 Urge drop of mercury, resting on the flat bottom of a vessel containing dilute 

 acid, changes it* form in a remarkable way when connected with one of the 

 uloBtrodm of > battery, the other electrode being placed in the acid. When 

 the mercury is made positive it becomes dull and spreads itself out; when it 

 made negative it gathers itself together and becomes bright again. M. Lipp- 

 raaim. who has made a careful investigation of the subject, finds that exceed- 

 ingly small variations of the electromotive force produce sensible changes in 

 the surface-tension. The effect of one Darnell's cell is to increase the tension 

 from 30-4 to 40'6. He has constructed a capillary electrometer by which 

 differences of electric potential less than O'Ol of that of a Daniell's cell can 

 be detected by the difference of the pressure required to force the mercury to 

 a given point of a fine capillary tube. He has also constructed an apparatus 

 in which this variation in the surface-tension is made to do work and drive 

 a machine. He has also found that this action is reversible, for when the 

 ana of the surface of contact of the acid and mercury is made to increase, 

 an electric current passes from the mercury to the acid, the amount of elec- 

 tricity which passes while the surface increases by one square centimetre being 

 sufficient to decompose '000013 grammes of water. 



Ux THE FORMS OF LIQUID FILMS WHICH ARE FIGURES OF REVOLUTION. 



A Spherical Soap-bubble. 



A soap-bubble is simply a small quantity of soap-suds spread out so as 

 to expose a large surface to the air. The bubble, in fact, has two surfaces, 

 an outer and an inner surface, both exposed to air. It has, therefore, a certain 

 amount of surface-energy depending on the area of these two surfaces. Since 

 in the case of thin films the outer and inner surfaces are approximately equal, 

 we shall consider the area of the film as representing either of them, and 

 shall use the symbol T to denote the energy of unit of area of the film, both 

 surfaces being taken together. If T is the energy of a single surface of the 

 liquid, T the energy of the film is 27". When by means of a tube we blow 

 air into the inside of the bubble we increase its volume and therefore its 



