The method of measuring capillary attraction, by as certaining the altitudes to which different fluids will rise in a glass tube, is not only liable to numerous objec tions, and to much uncertainty in the results, but is ap plicable only to the single substance of glass. We are thus left completely in the dark respecting the action which all the other solids exert upon fluid substances.
In order to remedy this defect, an instrument of very general application was sonic time ago proposed by Dr Brewster, the object of which was to measure, upon an optical principle, the diameter of the elevated circle of fluid, which any solid raises above the general level. Taus, in Plate CX. Fig. 8. let \INOP be the plan of a vessel filled with anv fluid, A toe section of a solid cy lindeT of any substance immersed in the fluid and at right angles to its surface, and BC the diameter of the circle of fluid, which is elevated by the action of the solid A. "'This elevated fluid is not terminated abruptly by the circle BC, but there is obviously a certain dis tance from A, WilCre the ordinate of the curve becomes a perceptible quantity, tither when examined by t ,e eye r by a micloscope. The apparent boundary of toe circle at li is ch. termined Isy obserNing the image of two delicate paranul fibres fixed ia a from( .A 1). as seen by reflection from the surf .cc of fluid at 13, to an eye at 1', assisted by a powerful inierosc.ope, adjusted to a distance equal to EB BD. When this image is seen by reflection from any part of the fluid surface without the circle BC, it will suffer no change of form ; but when it is seen by reflection from any portion of the elevated fluid, the fibres will appear distorted, and they will therefore indicate, by their return to a rectilineal form, the apparent termination of the circle BD. The same observation is made at 1), on the other side of A ; and a measure is thus obtained of the circle BC, by means of the micrometer screw, by which the micro scope at L, and the frame at 1), are moved along the sides of the vessel. By this instrument, which has been constructed and tried with stuxess, we can employ solids of all kinds, and ascertain their action upon fluids, un der circumstances which could not be obtained with ca pillary tubes. This method has another great advan tage, as it is extremely easy to remove from the out sides of the solids all that grease and foreign matter which it is so difficult to remove in tubes of glass. In another part of our work, we hope to be able to give a complete drawing of this instrument, and a table of re sults for various bodies. See Dr Brewster's Treatise
on .Vew Philosophical Intsruments, for -various purposes in the ?rts and Sciences, Book i. Edin. 1812.
Theory of Capillary Attraction.
Dr Hooke, who was one of the earliest writers on ca pillary attraction, ascribed the ascent of fluids, in capil lary tubes, to the unequal pressure of the atmosphere, arising front a diminution of the pressure of the air in consequence of its friction in the tube. This opinion was maintained till the experiment was tried in the receiver of an air-pump, and when the fluid was found to rise as high in vacuo as in the open air, a new cause was sought for the phenomenon. Sir Isaac Newton and Al r Ilauks bee were of opinion, that the attraction of the tube was insensible at sensible distances. Dr Jul in ascribed the suspension of the fluid to the attraction of the ring of glass to which the upper surface of the water is contigu „ous, and adheres. Dr Hamilton and Dr Matthew Young maintained that the fluid was elevated by the lower ring or glass contiguous to the bottom of the tube, and that this ring raises the portions of fluid immediaely below it, and then the other portions in succession, till the co lumn thus elevated was in equilibrium with the attraction of the ring.
Clairaut had the honour of being the first mathema tician who gave any tiling like a theory of capillary at traction. After pointing out the insufficiency of prece ding theories, he enters into an of all the forces by which the fluid is suspended in the tube, of which we shall endeat our to give our readers a brief account. Let A BC I) EF(111 (Pnae C X. Fig. 9.) he the section of a capillary tithe. Al N P the surface of the water in the vessel, 1 i the height of its ascent. VIZ the concave sur face of the fluid column, alai IKLAI an inde5nitely small column of fluid rcachitei; to the surface rt Al. Now t..c column AIL is solicited by the force of gra vity which acts through the ss oole extent of the column, and by the reciprocal attraction of the molcculx, woich, though they act the same in all the points of tl.e column. °oh exhibit their effects towards the extremitv Al. If any particle c is taken at a less distance from the surface than the distance at which the attraction of the liquid generally terminates, and if m It is a plane parallel to AIN, and at the same distance from the particle e, then this particle will be equally attracted by the water between the planes AIN, nzn. The water, however, below inn, will attract the particle downwards, and this effect will take place as far as the distance where the attraction ceases.