Capillary Tubes, in Physics, are very small pipes, whose canals or bores are exceedingly narrow, their usual diameter not exceeding one-twentieth or one-thirtieth of an inch, and in some cases being made so small as to be scarcely perceptible. One of the most singular phenomena of these tubes is, that if they are left open at both ends, and one end immersed in water, the water will rise in the tube to a considerable height above the surface of that into which they are immersed, the height being inversely as the diameters of the tubes. Different fluids, however, attain different heights, and quicksilver does not rise, but, on the contrary, stands higher outside the tube than within. Various hypotheses have been invented to account for this ascent of fluids: at the present day it is attributed to a species of attraction, supposed to exist between the fluid and the tubes, and which is hence denominated capillary attraction.