in a general sense, an appellation given to things on account of their extreme fineness or resembling hair.

CAPILLARY Tubes, in Physics, are small pipes of glass, whose canals are extremely narrow, their diameter being only a half, a third, or a fourth of a line.

The ascent of water, &c. in capillary tubes, is a phenomenon that has long embarrassed the philosophers; for let one end of a glass tube open at both extremities be immersed in water, the liquor within the tube will rise to a considerable height above the external surface: or if two or more tubes are immersed in the same fluid, one a capillary tube, and the other of a larger bore, the fluid will ascend higher in the former than in the latter; and this will be in a reciprocal ratio of the diameters of the tubes.

In order to account for this phenomenon, it will be necessary first to premise, that the attraction between the particles of glass and water is greater than the attraction between the particles of water themselves: for if a glass tube be placed in a position parallel to the horizon, and a drop of water be applied to the under side of the tube, it will adhere to it: nor will it fall from the glass till its bulk and gravity are far increased, as to overcome the attraction of the glass. Hence it is easy to conceive how sensibly such a power must act on the surface of a fluid, not viscid, as water, contained within the small cavity or bore of a glass tube; as also that it will be proportionably stronger as the diameter of the bore is smaller; for it will be evident that the efficacy of the power is in the inverse proportion of the diameter, when it is considered, that such particles only as are in contact with the fluid, and those immediately above the surface, can effect it.

Now these particles form a periphery contiguous to the surface, the upper part of which attracts and raises the surface, while the lower part, which is in contact with it, supports it: so that neither the thickness nor length of the tube is of any consequence here; the periphery of particles only, which is always proportional to the diameter of the bore, is the only acting power. The quantity of the fluid raised will therefore bear as the surface of the bore which it fills, that is, as the diameter; for otherwise the effect would not be proportional to the cause, since the quantities are always as the ratio of the diameters; the heights therefore to which the fluids will rise in different tubes, will be inversely as the diameters.

Some doubt whether the law holds throughout, of the ascent of the fluid being always higher as the tube is smaller: Dr Hook's experiments, with tubes almost as fine as cobwebs, seem to show the contrary. The water in these, he observes, did not rise so high as one would have expected. The highest he ever found it, was at 21 inches above the level of the water in the basin; which is much short of what it ought to have been by the law above mentioned. See Cohesion.

CAPILLARY Vessels. Many small vessels of animal bodies have been discovered by the modern invention of injecting the vessels of animals, with a coloured fluid, which upon cooling grows hard. But though most anatomists know the manner of filling the large trunks, few are acquainted with the art of filling the capillaries. Dr Monro, in the Medical Essays, has given what after many trials he has found most successful. See Injection.

CAPILLUS veneris. See ADIANTUM, Botany Index.