The magnitude of drops of fluids depends also upon the form of the surface from which they fall. If the fluid is collected at the extremity of a very minute fibre of glass, the drop will fall when its weight balances the attractive force exerted by the glass, and therefore, in the present case, the drop will be very small ; but if the fluid is col lected on a hemispherical surface, the surface of glass which is in contact with the fluid is greater ; and, there fore, the drop must contain a much greater quantity of water before its weight balances the attractive force of the hemispherical surface.
The form of a drop of fluid, abstracting the considera tion of its weight, would always be that of a perfect sphere ; and we accordingly find that the drops of rain by which the rainbow is formed, and very small drops of fluids lying upon a surface which does not attract them, have a shape almost perfectly spherical. In other cases the form of a drop is modified by its weight. Dr Young has given the following as the equation of the surface of a drop of water : In order to shew that two drops of water do not attract each other when at a distance, M. Monge put some spirit of wine into a cup, and having taken a capillary tube con taining some of the same fluid, he allowed it to fall from a height of a few lines, drop by drop, into the cup ; the drops did not immediately mix with the rest of the fluid, but preserved their form, which was nearly spherical ; roll ed over the surface with great freedom, like balls over a billiard table, impinged against each other ; changed their form by the force of impact ; and, after being reflected from each other, continued to move upon the surface till they were again mixed with the general mass. This ex periment does not succeed so well when the spirit of wine is warm. M. Mange explains this phenomenon by sup posing that a thin film of air adheres to the drop ; and, by diminishing its specific gravity, causes it to float upon the fluid surface ; and hence he concludes, that the experi ment will succeed best with those liquids which are most evaporable, or which have the greatest affinity for the sur rounding air. A similar phenomenon, as M. Alonge ob serves, is seen in the drops of water which fall from the oars during the rowing of a boat, and in the drops pro duced by the condensation of the steam of any warm fluid, such as coffee, &c. These drops are real spheres of fluid, and not spherical vesicles like those formed on the sur face of water with heavy rains. These results are hostile to the idea of 1\I. Saussure, who, in his Essays on Hygro metry, has stated that drops ol the same liquid cannot he pushed against one another, nor remain simply in contact without instantly uniting ; and that only hollow vesicular globules are capable of floating upon the surface of the same fluid with themselves.
In repeating the experiments of Menge, Dr Brewster found that the appearances were most beautiful when the capillary tube dischared the drops upon the inclined plane of fluid, which is elevated by the attraction of the edge of the cup. They ran down the inclined plane with great velocity, and sometimes even ascended the similar plane on the opposite side of the vessel. When the drop was discharged at the distance of one or two-tenths of an inch from the surface of the water, they had always the same magnitude when the tube was held in the same position ; but when the point of the tube was brought within a tenth of an inch of the surface of the spirit of wine, this surface, instead of attracting the drop to it instantly, as Saussure would have predicted, actually resisted the gravity or weight of drop, and allowed it to attain a diameter nearly twice as great as it would have had, if it had been discharg ed in the ordinary manner. This swoln globule floated upon the surface in the same manner as the smaller drops, surrounded with a depression of the fluid surface similar to what is produced by a glass globule floating on mer cury, or by the feet of particular insects, that have the power of running upon the surface of water. (See Fig. 5.) The floating globules arc often produced even when they are discharged from a height of three or four inches ; and by letting them fall upon the inclined plane of fluid formerly mentioned, they will often rebound from the sur face, and fall over the sides of the cup.
\Vhen a drop of mercury is laid upon glass, it assumes a flat spheroidal form, in consequence of its weight. The section of its surface, as AL La Place observed, by a ver tical plane drawn through its centre, is very much curved at its summit. The curvature increases on receding from that point, till the tangent to the curve is vertical. At this point, the curvature and the width of the section will be a maximum. Below that point it will approach its axis, and will at last coincide with the plane of the glass, and form with it an acute angle. M. Gay Lussac observ ed at the temperature of t2°.8 of the centigrade thermo meter, the thickness of a large drop of mercury, circular, and a decimeter in diameter, resting upon a plane surface of white glass perfectly horizontal. By a very accurate micrometer, he found its thickness to be 3.378 millimetres. M. Segner had long before obtained nearly the same result, viz. 3.40674 millimetres.