CAPILLARY ACTION. 573 



is only two or three millimetres, the surface-current will sweep away the whole 

 of the water, leaving a dry spot where the alcohol was dropped in. This 

 phenomenon was first described and explained by Professor James Thomson, 

 who also explained a phenomenon, the converse of this, called the "tears of 

 strong wine." 



If a wine glass be half-filled with port wine the liquid rises a little up 

 the side of the glass as other liquids do. The wine, however, contains alcohol 

 and water, both of which evaporate, but the alcohol faster than the water, so 

 that the superficial layer becomes more watery. In the middle of the vessel 

 the superficial layer recovers its strength by diffusion from below, but the film 

 adhering to the side of the glass becomes more watery, and therefore has a 

 higher surface-tension than the surface of the stronger wine. It therefore creeps 

 up the side of the glass dragging the strong wine after it, and this goes on 

 till the quantity of fluid dragged up collects into a drop and runs down the 

 side of the glass. 



The motion of small pieces of camphor floating on water arises from the 

 gradual solution of the camphor. If this takes place more rapidly on one side 

 of the piece of camphor than on the other side, the surface-tension becomes 

 weaker where there is most camphor in solution, and the lump, being pulled 

 unequally by the surface-tensions, moves off in the direction of the strongest 

 tension, namely, towards the side on which least camphor is dissolved. 



If a drop of ether is held near the surface of water the vapour of ether 

 condenses on the surface of the water, and surface-currents are formed flowing 

 in every direction away from under the drop of ether. 



If we place a small floating body in a shallow vessel of water and wet 

 one side of it with alcohol or ether, it will move off with great velocity and 

 skim about on the surface of the water, the part wet with alcohol being always 

 the stern. 



The surface-tension of mercury is greatly altered by slight changes in the 

 state of the surface. The surface-tension of pure mercury is so great that it 

 is very difficult to keep it clean, for every kind of oil or grease spreads over 

 it at once. 



But the most remarkable effects of change of surface-tension are those pro- 

 duced by what is called the electric polarization of the surface. The tension 

 of the surface of contact of mercury and dilute sulphuric acid depends on the 

 electromotive force acting between the mercury and the acid. If the electro- 



