.1864.] 



Prof. Guthrie on Drops, 



465 



sity and chemical diversity of the solid have only a small effect upon the 

 drop-size. The conditions of the experiment were similar to those pre- 

 viously described. 



Table X.—JFaier. 



T=22°'9C. 

 r=7'l millims. 

 Number of drops=20. 



O tl U o Ld/ll OU . 



Weight of 

 20 drops. 



Mean weight 

 of single drop. 





grms. 



grm. 







1^2-4846 ^ 









2-4848 





Cork 



■ 



2-4832 

 2-4789 ' 



0-12418 







2-4821 









[ 2-4877 j 









^2-5930 ^ 









2-5985 





Glass 



■ 



2-5989 

 2-5949 f 



0-12975 







2-5953 









^ 2-5900 j 









r 2-6225^ 









2-6229 









2-6260 

 2-6295 ' 



0-13118 







2-6296 









, 2-6116 j 





When a liquid drops from a solid it is not always that the adhesion be- 

 tween the solid and liquid is overcome. The phenomenon of " wetting " 

 implies a superiority of the adhesion between the solid and liquid over the 

 cohesion of the liquid ; and in all cases where a liquid drops from a solid 

 which it wets, the act of separation is a disruption of the liquid, and not a 

 separation of the liquid from the solid ; that is, the separation of the drop 

 is a failure of cohesion and not of adhesion. We are not, however, justified 

 on this account in anticipating that the size of a drop is unaffected by the 

 chemical nature of the solid from which it drops, even in those cases where 

 the adhesion between the solid and liquid is greater than the cohesion of 

 the liquid (that is, where the liquid completely wets the solid), because, 

 although it is the liquid which is broken, yet the size of the broken-off 

 part, or drop, depends in great measure upon the thickness of the residual 

 film, as we have seen in examining the influence of the growth-time (in 

 Part I.) and of the radius of curvature. 



Adhesion may also exist between a solid and a liquid which does not wet 

 it, as when a drop of mercury hangs from a glass sphere. But the cohe- 

 sion of the liquid in such a case, by its effort to bring the liquid to the 

 spherical form, and the weight of the drop so modify the adhesion between 



