COHESION OF FLUIDS. g'J 



We may suppose the particles of liquids, and probably those Law of super- 

 of solids also to possess that power of repulsion whidi has been ^'^J^ cohesion, 

 demonstratively shown by Newton to exist in aeriform fluids, 

 and which varies in the simple inverse ratio of the distance of 

 the particles from each other. In airs and vapours this force 

 appears to act uncontrolled ; but in liquids, it is overcome by 

 cohesive force, while the particles still retain a power of mov- 

 ing freely in all directions ; and in solids the same cohesion is 

 accompanied by a stronger or -weaker resistance to all lateral 

 motion, which is perfectly independent of the cohesive force, 

 and which must be cautiously distinguished from it. It is 

 simplest to suppose the force of cohesion nearly or perfectly 

 constant in its magnitude, ihi-oughout the minute distance to 

 which it extends, and owing its apparent diversity to the con- 

 trary action of the repulsive force, which varies with the dis- 

 tance. Now in the internal parts of a liquid these forces hold 

 each other in a perfect equilibrium, the particles being brought 

 so near that the repulsion becomes precisely equal to the 

 cohesive force that urges them together : but whenever there ^ 



is a curved or angular surface, it may be found by collect- 

 ing the actions of the different particles, that the cohesion 

 must necessarily prevail over the repulsion, and nust urge 

 the superficial parts inwards with a force proportionate to the 

 curvature, and thus produce the effect of a uniform tension 

 of the surface. For, if we consider the ef!'ect of any two 

 particles in a curved line on a third at an equal distance 

 beyond them, we shall find that the result of their equal 

 attractive forces bisects the angle formed by the lines of di- 

 rection; but that the result of their repulsive forces, one of 

 which is twice as great as the other, divides it in the ratio of 

 one to two, forming with the former result an angle equal to 

 one-sixth of the whole ; so that the addition of a third force is 

 necessary in order to retain these two results in equilibrium ; 

 and this force must be in a constant ratio to the evanescent 

 angle which is the measure of the curvature, the distance of the 

 particles being constant. The same reasoning may be applied 

 to all the particles which are within the influence of the cohe- 

 sive force ; and the conclusions are equally true if the cohe- 

 sion is not precisely constant, but varies less rapidly than the 

 repulsion. 



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