470 



Prof. Guthrie on Drops. 



[Recess, 



effects. But if there be a similar distinction between the two kinds of co- 

 hesion of hquids, as above pointed out in the case of sohds, we have the 

 following consequence. It is the persistent cohesion which causes the as- 

 sumption of the spherical form, the stubborn which resists the separation 

 of the drop. The former tends to diminish, the latter to increase its size. 

 As one or other predominates, the size of the drop varies. 



Accordingly the drop -size is by no means a measure of what is generally 

 called the cohesion of the liquid, but rather a measure of the difference 

 between the two cohesions, stubborn and persistent ; and the law is, that 

 the drop-size varies inversely as the persistent, and directly as the stubborn 

 cohesion of the liquid. 



In mercury, water, and glycerine the stubborn cohesion is greater in 

 proportion to the persistent cohesion than in the other liquids examined ; 

 but it by no means follows that persistent cohesion is wanting in mercury 

 or stubborn in alcohol. 



When a drop is in the act of falling its stubborn cohesion is in equili- 

 brium with the resultant of two forces — the one, the persistent cohesion, 

 tending to produce a spherical form, the other the weight of the drop. 

 Since the former of these component forces is, for the same liquid, constant, 

 it seems as though the weight of the drop might be taken as a measure 

 and expression of the stubborn cohesion. But such is not the case, because 

 we have no ground for supposing that the diameter of the drop where the 

 separation occurs is of constant size ; on the contrary, it must be conceded 

 that in larger drops this hypothetical surface of stubborn cohesion is larger 

 than in smaller drops. Further, unless we know the exact shape of a drop 

 in all cases, we are not in a position to deduce the size of the surface of 

 cohesion from the drop -size or drop- weight. 



In the cases where it has been tried, it has not been found that the nature 

 of the gaseous medium in the case of SLG exerts any appreciable or defi- 

 nite influence upon the drop-size. Taking glass for the solid and water 

 for the liquid, the medium was changed from, air to nitrogen, hydrogen, 

 and carbonic acid. The exceedingly slight variation wrought in the 

 drop-size by this change may probably have been due to the different solu- 

 bility of the gases in water, and the consequent alteration in the cohesion 

 of that liquid. 



Having now traced the effect of variation in the conditions which deter- 

 mine the size of a drop in the general case SLG (or where from, a solid a 

 liquid drops through a gas), we come to the case SLL (that is, where from 

 a solid a liquid drops through a liquid). As in the cases of SLG, we must 

 here also take the three terms of such chemical nature as to be without 

 action upon one another. 



SLL. Fro7n a Solid a Liquid drops through a Liquid. 

 A preliminary quantitative experiment was made under the following 

 conditions :-— Water was made to drop from a glass sphere at the rate 



