Capillary action (or capillarity) is “the action by which the surface of a liquid where it contacts a solid is elevated or depressed, because of the relative attraction of the molecules of the liquid for each other and for those of the solid.”
The Italian polymath Leonardo di ser Piero da Vinci discovered capillary action sometime around 1490-1499 or 1503-1505. He observed that when a tube was dipped in water, the water rose up the tube.
Here is a photograph displaying capillary action clearly:
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Without capillary action, the liquid level in all the tubes would be the same. Smaller diameter tubes have more relative surface area inside the tube, allowing capillary action to pull the liquid up higher than in the larger diameter tubes.
Here is a portrait of Leonardo da Vinci:
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- https://books.google.ae/books?id=-tVyDwAAQBAJ&dq=leonardo+da+vinci+a+reference+guide+to+his+life+and+works&source=gbs_navlinks_s, page 73.