66 
Dr. Young’s Essay 
of this theory with the phenomena, and to inquire afterwards 
for its foundation in the ultimate properties of matter. But it is 
necessary to premise one observation, which appears to be 
new, and which is equally consistent with theory and with 
experiment ; that is, that for each combination of a solid and a 
fluid, there is an appropriate angle of contact between the 
surfaces of the fluid, exposed to the air, and to the solid. This 
angle, for glass and water, and in all cases where a solid is 
perfectly wetted by a fluid, is evanescent : for glass and mer- 
cury, it is about 140°, in common temperatures, and when the 
mercury is moderately clean. 
II. Form of the Surface of a Fluid. 
It is well known, and it results immediately from the com- 
position of forces, that where a line is equably distended, the 
force that it exerts, in a direction perpendicular to its own, is 
directly as its curvature ; and the same is true of a surface of 
simple curvature ; but where the curvature is double, each 
curvature has its appropriate effect, and the joint force must be 
as the sum of the curvatures in any two perpendicular direc- 
tions. For this sum is equal, whatever pair of perpendicular 
directions may be employed, as is easily shown by calculating 
the versed sines of two equal arcs taken at right angles in the 
surface. Now when the surface of a fluid is convex externally, 
its tension is produced by the pressure of the particles of the 
fluid within it, arising from their own weight, or from that of 
the surrounding fluid ; but when the surface is concave, the 
tension is employed in counteracting the pressure of the at- 
mosphere, or, where the atmosphere is excluded, the equivalent 
pressure arising from the weight of the particles suspended 
