CAPIIXABY ACTION. 



and this, whether the atmosphere exerts any pressure or not. The force thus 

 produced by tbe introduction of a drop of water between two plates is enormous, 

 and i. often wfficient to press certain parts of the plates together so power- 

 fbOy M to bruise them or break them. When two blocks of ice are placed 

 loosely together so that the superfluous water which melts from them may 

 drain away, the remaining water draws the blocks together with a force suffi- 

 cMOt to cause the blocks to adhere by the process called Regelation. 



In many experiments bodies are floated on the surface of water in order 

 that they may be free to move under the action of slight horizontal forces. 

 Thus Newton placed a magnet in a floating vessel and a piece of iron in 

 another in order to observe their mutual action, and Ampere floated a voltaic 

 battery with a coil of wire in its circuit in order to observe the effects of 

 the earth's magnetism on the electric circuit. When such floating bodies come 

 near the edge of the vessel they are drawn up to it, and are apt to stick 

 fast to it. There are two ways of avoiding this inconvenience. One is to grease 

 the float round its water-line so that the water is depressed round it. This, 

 however, often produces a worse disturbing effect, because a thin film of grease 

 spreads over the water and increases its surface-viscosity. The other method 

 is to fill the vessel with water till the level of the water stands a little 

 higher than the rim of the vessel. The float will then be repelled from the 

 edge of the vessel. Such floats, however, should always be made so that the 

 section taken at the level of the water is as small as possible. 



PHENOMENA ARISING FROM THE VARIATION OF THE SURFACE-TENSION. 



Pure water has a higher surface-tension than that of any other substance 

 liquid at ordinary temperatures except mercury. Hence any other liquid if 

 mixed with water diminishes its surface-tension. For example, if a drop of 

 alcohol be placed on the surface of water, the surface-tension will be diminished 

 from 80, the value for pure water, to 25, the value for pure alcohol. The 

 surface of the liquid will therefore no longer be in equilibrium, and a current 

 will be formed at and near the surface from the alcohol to the surrounding 

 water, and this current will go on as long as there is more alcohol at one 

 jart of the surface than at another. If the vessel is deep, these currents will 

 be balanced by counter currents below them, but if the depth of the water 



