1881.] 



On Pendent Drops. 



3G9 



with which, it is in contact. The extent to which this portion of the 

 cohesion is called into play in sustaining the weight of the drop, is 

 measured by the amount of negative curvature above-mentioned. 



Direct evidence of this negative curvature is easily obtained by allow- 

 ing a drop to hang from the end of an open vertical tube of narrow bore, 

 into which a supply of liquid is led by a fine capillary thread ; as the 

 drop grows the head of liquid supported may be observed to diminish. 

 Owing, however, to the smallness of the change, the slowness with 

 which the meniscus in a fine tube adjusts itself, and the liability to 

 alteration of the surface tension of the drop through long exposure, 

 the method cannot be used with advantage for measuring the curva- 

 ture. 



It is important, however, to notice that there is not any physical 

 breach of continuity, or critical point reached, when the pressure at 

 the base becomes zero or negative, which can influence the separation 

 of the drop. For there is no reason to suppose that the small negative 

 pressure finally attained at the base of the drop, varying as it does 

 with the diameter of the tube, represents any physical limit of the 

 cohesion, which is probably very great. 



Hence, it is clear that the separation of the drop is to be attributed 

 to the surface becoming unstable for small oscillations, rather than to 

 the cohesion being in any true sense overcome by the force of gravity, 

 and that the cleavage of the drop is analogous to the cleavage of the 

 unduloid or cylinder first studied experimentally by M. Plateau. 



It is necessary to insist on this point, for it has been apparently 

 misconceived by Professor Quincke, who has endeavoured to deduce 

 the value of the tension by equating the weight of the drop which 

 falls to the tension multiplied by the circumference of the tube or 

 cylinder from which it hangs, the latter being taken so small that the 

 drop hangs with the uppermost element of the liquid vertical. (See 

 fig. 3.) 



The nature of his proceeding will be best seen from the following 

 quotation.* 



Fig. 3. 



* From a paper communicated to the Koyal Academy of Sciences at Berlin, 

 May 28, 1868, and translated in " Phil. Mag.," 1869, by Professor Jack. The 

 italics are my own. 



