cohesion.

It acts only at insensible distances and is strictly a molecular force. It is that force which prevents solid bodies from falling apart. Liquids like molasses and honey possess more cohesive force among the molecules of which they are composed than limpid liquids like water and alcohol. The former are said to be viscous, or to possess viscosity.

21. Adhesion. That force which causes unlike substances to cling together is called adhesion. It is that force which keeps nails, driven into wood, in their places. You can climb a pole because of the adhesion between your hands and the pole. We could not pick anything up if it were not for adhesion. Glue, when dry, possesses both cohesion and adhesion to a great degree.

22. Capillarity. Examine the surface of water in a vessel. You find the surface level, except around the edge next the glass, as at A (Fig. 7.)

Fig. 7.

Fig. 8.

1. Thrust vertically into water three glass tubes, A, B and C (Fig. 8), open at both ends. You notice the water ascends in each to a different height, and that the ascension varies inversely as the diameter of the bore; i. e., the smaller the bore, the higher the water ascends.

2. Seal one of the tubes at its upper end. The water enters but little, as shown at D (Fig. 8), on account of the resistance of the air pressure within the tube.

3. Thrust vertically two plates of glass into water, and gradually bring the surfaces near to each other. Soon the water rises between the plates, and rises higher as the plates are brought nearer. If their surfaces be mutually parallel and vertical, the water rises to the same height at all points between the plates, as shown at A (Fig. 9.)

Fig. 9.

4. If the plates be united by a hinge, and form an angle, the height to which the water ascends increases as the distance between the plates decreases up to their line of junction, where it attains a maximum, as shown at B (Fig. 9.)

5. Decrease the angle between the plates, and the water ascends higher, as shown at C (Fig. 9.) Thus it is seen that the ascension varies inversely with the angle between the plates; i. e., the smaller the angle, the higher the water ascends.

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.

24. Absorption of Gases by Liquids depends on molecular attraction and motion. Water at a temperature of 0° cen. (32° f.), is capable of condensing in its pores (17) six hundred times its own bulk of ammonia gas. The absorption of oxygen from the air causes some oils to become more viscous, to eventually become solid, without losing in weight, in fact sometimes gaining. Other oils dry up, or evaporate, leaving little or no residue.

25. Force.—Force is that which can produce, change or destroy motion.

We see a body move; we know there must be a cause; that cause we call force. We see a body in motion come to rest; this effect must have had a cause; that cause we attribute to force. The forces acting in machines are distinguished into driving and resisting forces. That component of the force which does the work is called the "effort."

26. Friction is usually a resisting (29) force, tending to destroy motion; but it is sometimes the means of the transmission of motion.

27. Work is the result of force acting through space. When force produces motion, the result is work. Work is measured by the product of the resistance into the space through which it is overcome.

28. Energy, which is defined[6] as the capacity for doing work, is either actual or potential. Actual or kinetic energy is the energy of an actually moving body, and is measured by the work which it is capable of performing while being brought to rest under the action of a retarding force.

Potential Energy is the capacity for doing work possessed by a body in virtue of its position, of its condition, or of its intrinsic properties. A bent bow or a coiled spring has potential energy, which becomes actual in the impulsion of the arrow or is expended in the work of the mechanism driven by the machine. A clock weight, condensed air and gunpowder are examples.

This form of energy appears in every moving part of every machine and its variations often seriously affect the working of machinery. (84.)