CAPILLARY tubes, in physics, litfle pipes, whose canals are extremely nar row, their diameter being only a half, third, or fourth of a line.
The of water, &c. in capillary tubes, is a phenomenon that has long em barrassed philosophers ; for let one end of a glass tube, open at both ends, be im merged in water, and the liquor within the tube will rise to some sensible height above the external surface ; or if two or more tubes are immerged in the same fluid, one of them a capillary one, the other of a larger bore ; the fluid will as cend higher in the capillary tube than in the other, and this in the reciprocal ratio of the diaMeters of the tubes.
In order to account for this phenome non,itwill be necessary first to premise that there is a greater attraction between the particles of glass and water, than there is between the particles of water them selves: this appears plain from experience which proves the attractive power in the surface of glass to be very strong; whence it is easy to conceive how sensible 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 in proportion stronger as the diameter of the bore is smaller ; for that the efficacy of the power follows the inverse proportion of the diameter is evident from hence, that only such par ticles as are in contact with the fluid, and these immediately above the surface, can affect it. Now these particles form a
periphery, contiguous to the surface, the upper part of which attracts and raises the surface, and the lower part, which is in contact with it, supports and holds it up, so that neither the thickness nor length of the tube avails any thing, only the said periphery of particles, which is always proportional to the diameter of the bore: the quantity of the fluid raised will therefore be as the surface of the bore which it fills, that is, as the diameter; as the effect would not be otherwise pro portional to the cause, since the quanti ties follow the ratio of the diameters, the heights to which the fluids will rise in, different tubes will be inversely as the diameters.
Some, however, 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 skew the contrary. The water in these, he observes, did not rise so high as one would have expected. The highest he ever found was at 21 inches above the level of the water in the basin, which is much short of what it onght to have been by the law above mentioned.