When a glass capillary tube is dipped in water, the water rises up in the tube. This is true of all liquids which wet the capillary tube.
When a glass capillary tube is dipped in mercury, the mercury is depressed below the free surface of the liquid in the container. This is true of all liquids which do not wet the capillary tube. The narrower the bore of the tube, greater is the rise or fall of the liquid in the tube.
The phenomenon of rise or fall of a liquid in a capillary tube is known as capillarity.
Following are some illustrations of capillarity.
(i) A blotting paper soaks ink by capillary action. The pores of the blotting paper act as capillaries.
(ii) The action of towel in soaking up the moisture from hands or the body is due to capillary action of the cotton.
(iii) The oil in an oil wick rises up through the narrow spaces between the threads of the wick which act as fine capillary tubes.
(iv) Water is retained in a piece of sponge on account of capillarity.
(v) Walls get damped in rainy season due to the absorption of water by bricks by the capillary action.
(vi) A pen nib is split at the tip to provide a narrow capillary tube and the ink is drawn up to the point continuously.
(vii) Leaves, trunk and branches of a tree possess fine capillaries. Water rises even up to the topmost leaves by capillary action.