Capillary Action, or Capillarity, the tendency of liquids to rise or to be depressed in tubes of small diameter. Capillarity is due to the adhesion of the liquid to the sides of the tube, and to the surface tension of the liquid.

A liquid that wets a capillary tube will rise. If the liquid does not wet the tube it will be depressed. The smaller the diameter of the tube, the greater the elevation or depression of the liquid. As temperature increases, the amount of elevation or depression decreases.

Capillary action can be seen when a corner of a paper towel is touched to spilled water. The water soon spreads into other parts of the towel because loose fibers have spaces between them that act as capillary tubes. The drying action of a bath towel is also due to capillarity. Kerosene rises in the wick of a lamp through capillary action.

Compact soil has very small, continuous spaces through which water tends to rise by capillary action. When the water reaches the surface of the soil, it is lost into the air by evaporation.