6. When a drop of oil is placed between two glass plates arranged as shown at A (Fig. 10), if the surfaces are not too far distant, and if the oil touches both surfaces, it will be seen to work its way to the junction of the plates; showing that oil between surfaces has a tendency to flow towards the apex of the angle.
Fig. 10.
7. Place a drop of oil on a taper piece of metal, as shown at B (Fig. 10). The oil will gradually recede from the point to a place where there is more metal, showing that oil on surfaces has a tendency to flow towards the largest part.
Fig. 11.
8. When a drop of oil is placed between two watch glasses arranged with flat and convex sides adjacent, as at A (Fig. 11), or with convex sides adjacent, as at B (Fig. 11), if the glasses are rigidly fixed in their relative positions the drop of oil can be shaken from its location only with great difficulty; the oil at C holding its place with greater tenacity than the oil at D.
The foregoing phenomena are called capillary action, or capillarity. Capillary action is due to the forces of cohesion (20), and to the forces of adhesion (21.)
23. Centrifugal Force.—The tendency of a body rotating round a point to escape from that point is called centrifugal force.
Place a small quantity of oil on the arm of a balance, near the arbor. Rotate the wheel rapidly. The oil is seen to flow towards the rim of the wheel.