When we dip three tubes with fine bores but with different diameters into a clean water, we observe that water rises in the tubes but the narrower the bore the higher the height to which the rises.

Capillary action (sometimes capillaritycapillary motion, or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper, in some non-porous materials such as liquefied carbon fiber, or in a cell. It occurs because of inter molecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container act to lift the liquid. In short, the capillary action is due to the pressure of cohesion and adhesion which cause the liquid to work against gravity.

Capillarity or capillary action is the tendency of a liquid to rise or fall in a narrow tube.

In both the water and the soap solution, the surface of the liquid or its meniscus curves upwards. But in mercury the meniscus is curved downwards away from cohesion and adhesion will be required to explain this capillary action.

Cohesion is the force of attraction between molecules of the same kind, e.g. the molecules of water.

Adhesion is the force of attraction between molecules of different kinds, e.g. the molecules of water and glass.

Cohesion and adhesion explain the different action of water and mercury when spilled on a clean glass surface.

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