Exit. 23. If a drop of mercury be introduced into a conical capillary tube, held in a horizontal position, the mercury will move towards the wide end.
Exp. 24. By observing the surface of a column of mercury depressed in a capillary tube, or inclosed in a barometer tube, it will be found to be convex upwards. Air Ilatty has endeavoured to show, that this convexity differs very little from the form of a hemisphere. Dr T. Young maintains that this result is inaccurate, and that the angle formed by the surface of the mercury with the side of the tube is obtained this result, by ob serling in what position the light reflected from the mer curial surface began to reach the eye, and he has found it correct, from the C.Oniparison of a great variety of expe riments of different kinds. This ingenious philosopher has prosecuted this branch of capillary attraction with great abilityand success. lie has calculated the precise form of the surface of the mercury in a variety of cases. In order to confirm these calculations, he employed another method, which consists in finding the mass of the quantity of fluid, supported by the tension of the sur face at each concentric circle, and inferring from this the inclination of the curve to the horizon, assuming for the mean height the height of the external circumference of each portion ; a supposition which almost compensates for the omission of the curvature of its surface. As a specimen of these methods, we shall insert the following Tables, by means of which the curves may be correctly delineated. They arc suited to a central depression of 0.007.
Dr Young has embodied the results of these lations in the following formula, which gives the central depression without any perceptible error, 0.015d which is nearly half the versed sine 0.015 of a spherical surface, and then f =-- c — 14.5 This approximate formula supposes the surface to he spherical, and is corrected by a comparison with the results of the calculations, so as to agree with them all without an error of one two-thousandth of an inch in the most unfavourable cases. Dr Young has also found a formula, when the diameter of the tube is moderate, for spewing the difference between the central and marginal depression, which may be of the greatest service in correcting the height of the basometer, whe we have obtained a measure of the highest or lowest point of the surface 15(5d + + 18 If d were very large, it would require some farther cor reetion, being ultimately too great by 0.0069. As our
limits will not permit us to pursue this interesting sub ject any farther, we must refer our readers to Dr Voting's able paper on the Cohesion of Fluids, published in the Phd. Trans. for 1805, and in his Lectures on Xat. Phil. Yoh ii. p. 666-669.
Exp. 25. When the mercui y and the capillary tube are perfectly dry, the fluid will rise above the general level, like all other liquids. This was ascertained by Professor Casbois of Metz, who boiled the mercury se veral times, in order to lice it from all humidity, and expel any foreign particles. By drying the mercury and the tube to a very great degree, Messis La Place and Lavoisicr constructed barometers, in which the mercu rial column was terminated above by a plane surface, and they even succeeded in rendering the upper surface of the mercury concave. The observations given under Experiment 24 are referable to barometers constructed in the usual way.
Exp. 26. If two plates of glass be placed parallel to each other, at the distance of about of an inch, the water in ,which they are immersed will risc one inch above its level in the vessel ; ar.d when the plates are placed at different distances, the heights to which the water will rise, will be reciprocally proportional to the distances of the plates.
Exp. 27. If a capillary tube be taken of such a mag nitude, that the diameter of its bore is equal to the dis tance between the plates in the preceding experiment, the water will risc in it to the same height as between the plates. See Newton's Optics, p. 366.
Exp. 28. If a fluid is either elevated or depressed be tween two vertical and parallel planes, the planes will telid to approach each other.