In December 1805, M. le Comte La Place laid before the National Institute of France a Dissertation on Capilla ry Attraction, which is marked with the high genius of its distinguished author. In 1806, it was published under the title of Supplement au dixieme du Traiti de ilI•eu nique Celeste, sur Capillaire. In the theory ad vanced by Clairaut, it was supposed that the action of the capillary tube extended to a sensible distance, and even to the fluid particles in its axis ; and, instead of attempting to discover the law of that action, he contented himself with observing that there were an infinity of laws of attrac tion, which, if substituted in his formul?, would give re sults corresponding to those obtained from experiment. M. La Place, however, was anxious to ascertain the pre cise law of attraction which represented the phenomena ; and, after long researches, he at last succeeded in disco vering, that all the phenomena could be represented by the same law which represents the phenomena of refrac tion, namely, the law in which the attraction is only sensi ble at insensible distances; and upon this he has founded a rigorous theory of capillary attraction. The first Section of this work treats of the Theory of Capillary Attraction ; and the second contains its application to some of the ex periments of Hawksbee, and to others made at La Place's request by MM. Ilauy and Tremery. In the year 1807, La Place published his Supplement a la Theorie de • tion Capillaire ; the object of which was to perfect the theo ry which he had given—to extend its application—to con firm it by new comparisons with experimental results— and, in presenting under a new point of view the effects of capillary action, to chew the identity of the attractive force upon which it depends with those which produce chemical affinities.
This Supplement treats, 1. Of the fundamental Equation of the Theory of Capillary Actions. 2. Of a new manner of considering Capillary Attraction. 3. Of the Attraction and apparent Repulsion of small Bodies which move on the surface of Fluids. 4. On the Adhesion of Discs to the Surface of Fluids. 5. On the Figure of a large Drop of Mercury, and of the Depression of the Fluid in a Tube of Glass of a great Diameter. The theoretical results ob tained from the formula investigated by M. La Place agree, in a very wonderful manner, with a series of experi ments made, at his request, by M. Gay Lussac, as will be seen from the abstract of this theory with which the pre sent chapter is concluded.
Baying thus given a brief account of the progress of this branch of science, we shall now lay '.'..efore our readers such additional iuhOrmation on the cohesion of fluids and on capillary attraction, as may appear to be necessary for com pleting the view of the subject which might be expected in a work of this kind.
The experiments which were made during the last cen tury on the ascent of fluids in tubes of glass, differed so widely from each other in their results, that no confidence whatever could be placed in them. Some philosophers did not scruple to assign different heights for the same fluid and the same tube ;* and even if the proper mode of cleaning the tube had enabled them to observe the highest point to which the water rose, they had no correct method of measuring the difference of level between the summit of the elevated column and the surface of the fluid. The
rise of the water on the outside of the glass tube, ren dered it particularly difficult to make such a measure ment with the accuracy which such delicate observations required.
The first attempt to construct an accurate instrumcnt, appears to have been about the same time by Dr Brewster and M. Gay Lussac, who, without any knowledge of each other's invention, have employed the•very same principles for ascertaining, with the utmost accuracy, the altitude of the fluid above its natural level. We do not know which of these inventions are entitled to the merit of priority. Dr Brewster's instrument was invented some time in 1806. An account of it was submitted to the Royal Society of Edinburgh in February 1811, and a (hawing and descrip tion of it was published in our article CAPILLARY ATTRAC TION in 1812. Gay Lussac's instrument must have been invented in 1807, and probably much earlier, as the expe riments contained in La Place's second supplement, which appeared in that year, seem to have been made with it ; but so far as we know; no description of the instrument was published till 1816, when it appeared in M. Blot's Traite de Physique.
This instrument is represented in Plate CCCXVI. Fig. 3, where ABCD is a large cylindrical vessel of glass for containing the liquid. It is placed upon a base, which can be adjusted by the three adjusting screws V, V, V, so that its orifice AB may be perfectly horizontal, which can easily be ascertained by placing a level upon it. The capillary tube TSH with which the experiment is to be made, has a vertical motion in a groove CC, perpendicular to a plate a b, which is placed on the orifice AB of the glass vessel. By this means the tube is kept in a vertical position, and it is required only to measure the height of the column HS above the level NN of the fluid. For this purpose Gay Lussac employs a divided rule RR, which can be placed in a vertical position by means of two ad justing screws v v, and a plumb line FP. Along the di vided scale, a small telescope GH of a short focal length, and with cross wires in the focus of the eye-glass, is made to move by the screw nut s, so that the horizontal wire may be made just to touch the lowest point of the surface of the fluid. In order to determine the point H which corre sponds to the natural level of the fluid surface, the plate a b, along with the tube TT, is pushed aside till it is stop ped by the side of the glass vessel, (for if it were taken out, the surface NN would descend,) and the bar t t rest ing upon the plate a' b' is placed on the side of the vessel from which the tube has been moved. The bar t t is made to descend by a screw cut upon it, till the lower point t touches the water, which is known by the water rising in stantaneously round it. When this contact is effected, a small portion of the water is removed by the small mea• sure M, at the end of a crooked wire, so that the lower extremity t is above the surface of the liquid by a very small quantity. The horizontal wire of the telescope is then made to come into apparent contact with the end t of the bar, and the distance between this and the division cor responding to S, is a true measure of the altitude HS of the fluid.