464 



Prof. Guthrie on Drops. 



[Recess, 



drops. For several reasons, this plan of comparison is not sufficiently ac- 

 curate to measure drop-sizes ; but it offers a method of making the differ- 

 ence of drop-size visible to any number of persons at once. 



The only other variation in the geometrical relation between the solid 

 and the liquid, which we shall consider, is the variation in^the size of a cir- 

 cular horizontal plane from which drops fall. 



Five disks of copper foil were cut of the radii -^-^j, -f-^, -f^, -J^th of an 

 inch respectively. These were fastened horizontally to vertical wires, and, 

 having been thoroughly cleaned by momentary immersion in nitric acid 

 and washing, water was made to drop from them at the rate gt—2^\ 



Table IX. shows the influence of this kind of variation upon drop-size. 

 The want of accord in the numbers of the largest disk is owing to a pecu- 

 liar tremor which the drops exhibit at the moment of delivery. The same 

 phenomenon was noticed, but to a less extent, with the next smaller disk. 

 "With the remainder it was not noticed. 



The curvature and shape of the solid, and its consequent massing towards 

 the liquid, is intimately connected with the next phase of variation which 

 we shall consider, to wit, the variation in the chemical composition of the 

 solid from which the drop falls. The influence of this kind of variation is 

 to be studied by examining the size of drops formed under like circum- 

 stances, from spheres of the same size, but made of different material. Since 

 in this case the liquid remains the same, we must limit the solids examined 

 to such as the liquid completely wets. In this case, variation in the drop- 

 size implies a variation in the thickness of the liquid film covering the 

 solid. The latter must be caused by variation in the adhesion between the 

 solid and liquid. Finally, such adhesion can only vary through one or both 

 of two causes — namely, variation in the density of the solid, or in its spe- 

 cific adhesion dependent upon its chemical nature. 



The first qualitative experiment was made upon three equal spheres of 

 brass, glass, and cork. They were hung one above the other in the manner 

 before described, so that the drop from one sphere fell upon the lower 

 one*. It was found that, in whatever order the spheres were arranged, 

 when the flow was uniform and not quicker than 2", the dropping from 

 the cork took place with the greatest rapidity, that from the glass next, 

 and that from the brass most slowly — showing that the brass gives rise to 

 the largest, the glass to the next largest, and the cork to the least drops. 

 From this it would seem that the drops are in the same order as to size as 

 are the soHds as to density. We shall find, however, that this is not always 

 the case, and that some other property as well as density is at work to in- 

 fluence the drop-size. The quantitative experiment, the results of which 

 are given in Table X., confirms the result of the qualitative experiment 

 given above, but shows, at the same time, that the joint influences of den- 



In this kind of experiment there should be a considerable mass of cotton wool on 

 each sphere to receive the di'ops from the higher one, and, by acting as a reservoir, to 

 regulate the flow. 



