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JDr. Young's Essay 
distance to which it extends, and owing its apparent diversity 
to the contrary action of the repulsive force, which varies 
with the distance. 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 col- 
lecting the actions of the different particles, that the cohesion 
must necessarily prevail over the repulsion, and must 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 effect 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 direction ; 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 mea- 
sure of the curvature, the distance of the particles being 
constant. The same reasoning may be applied to all the par- 
ticles which are within the influence of the cohesive force: and 
the conclusions are equally true if the cohesion is not precisely 
constant, but varies less rapidly than the repulsion. 
VII. Cohesive Attraction of Solids and Fluids. 
When the attraction of the particles of a fluid for a solid is 
less than their attraction for each other, there will be an 
