Rise or fall of liquid in a small passage or tube. When a glass tube of small internal diameter is inserted into water, the surface water molecules are attracted to the glass and the water level in the tube rises. The narrower the tube, the higher the water rises. The water is said to wet the tube. Water will also be drawn into the fibres of a towel, even if the towel is in a horizontal position. Conversely, if a glass tube is inserted into mercury, the level of the liquid in the tube falls. The mercury does not wet the tube. Capillarity is caused by the difference in attraction of the liquid molecules to each other and the attraction of the liquid molecules to those of the tube.
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