smallest; or the heights are inversely as the diameters. Water will be seen to rise in a similar way between two glass plates placed as fig. 5, with two of the upright edges touching, and the other two slightly apart. The sustained film rises higher as the plates approach, assuming the form of a particular curve. The fluid rises also slightly on the outside of the tubes and plates, and the surface of the sustained column within the tube is seen to be hollow like a cup (fig. 6). But liquids do not always ascend in narrow tubes or Fig. 6. Fig. 7. spaces; it is only when they wet the solid substance that they do so. If a greasy glass tube is dipped in water; or, still better, if a clean glass tube is dipped in mercury, the liquid inside, instead of rising, sinks below the general level; the surface of the column, too, becomes convex instead of concave (fig. 7). The rise or the depression depends upon the adjustment between the forces of adhesion and cohesion, as in the case of wetting. When the liquid wets the tube, the particles next its surface have part of their weight taken away or supported by adhesion, and thus a longer column is required to balance the pressure of the rest of the fluid. In cases where the cohesive attraction of the liquid particles within the tube for one another is too strong to permit them to adhere to its surface, that cohesion tends to draw them away from it, and downwards, so that a shorter column suffices to balance the general pressure. Capillary attraction is exemplified in many familiar appearances, and plays an important part in nature. If a piece of sponge or a lump of sugar be placed so that its lower corner touches the water, the fluid will rise up and wet the whole mass. When a dry wedge of wood is driven into the crevice of a rock, and afterwards moistened with water, it will absorb the water, swell, and sometimes split the rock. 35. Endosmose or Osmose.-Connected with capillary attraction is endosmose. If two liquids be separated by a piece of ox-bladder, the one below the membrane being pure water, the other above being a solution, say of carbonate of soda, the water will pass through the membrane against gravity, and raise the solution above its former level. A smaller portion of the solution finds its way into the water. This remarkable phenomenon is known as endosmose and exosmose, or simply as osmose. It is not yet fully understood, but is believed to play a part in the passage of the fluids through the membranes of living animals and plants. ENERGY. 36. Energy is defined as the capacity of doing work. A man does work when he makes something move against a force that resists his exertions; as when he forces a spade into the ground, lifts a spadeful of the soil some feet high, and throws it horizontally into a cart. Here the resisting forces are the cohesion and friction of the soil which oppose the entrance of the spade, gravitation which resists the vertical lifting, and the inertia of the mass which has to be overcome in the horizontal throw. It is to be carefully observed here that force may be applied without doing any work, as when we try in vain to lift a ton-weight. No work is done unless the resisting body is made to move over more or less space. 37. Work, how measured.-The simplest form of work, and that with which all other kinds are compared, is that of lifting a weight perpendicularly from the ground. The lifting of a pound-weight 1 foot high is taken as the unit of work, and is called the FOOT-POUND. If, then, we lift 5 lbs. 7 feet, we have done 5 times 7, or 35 units of work. Suppose now that the work is to be done against friction, as in dragging a heavy weight along a level road; we can measure in pounds-weight the pull necessary to begin the motion, and suppose it is 50 lbs. If, then, the weight is dragged 100 yards, the work done is 300 x 50 15,000 foot-pounds. = 38. Energies of Nature.-But there are other sources of energy beside the muscles of men and animals. We see the forces of nature constantly doing work. A stream of water is made to turn a wheel and grind corn or spin cotton; the wind propels ships; the energy of steam does half the work of the world. Let us attend more closely to the various shapes that the energies of nature put on. 39. Energy of moving Bodies.-We have seen that when a body falls freely, it will at the end, say of 4 seconds, have fallen 256 feet, and have acquired a velocity of 128 feet per second. Suppose, then, that a stone is projected upwards with a velocity of 128 feet; gravitation will now detract from its velocity every second as much as it added to it in descending; so that at the end of 4 seconds, it will have ascended 256 feet, and lost all its velocity. Here the impulse that the stone receives imparts to it an energy that enables it to lift its own weight 256 feet against gravitation; in other words, supposing it to weigh a pound, it has done 256 units of work. Were the stone projected horizontally, it might be made to do work in some other form. A body in motion, then, has energy, or the power of doing work. 40. Energy of a moving Body, how measured.-The amount of energy inherent in a moving body depends on two things. 1st, It depends on the quantity of matter; for if the stone had been only half a pound, the units of work would have been only the half of 256. 2d, It also depends on the velocity, but not in the simple ratio. For by the law of falling bodies (see par. 16), the space v2 passed over is as the square of the velocity, or 8 = 2g so that were the stone thrown with a velocity of twice 128 feet, it would ascend four times 256 feet. The energy then is as the mass multiplied by the square of mv2 velocity, or e= This is known as the vis viva of a 2g moving body. Energy never Annihilated, but only Transformed. 41. Energy of Two Kinds.-When the stone has ceased to move upwards, what has become of the energy it had at the outset? It has not perished; it has only taken another form. Suppose that by some contrivance the stone is arrested at its highest point, and left suspended; whenever it is detached, it will on its descent acquire the same velocity and therefore the same energy that it had when first projected upwards. The energy that a body has when in motion is called kinetic energy; when a body is so situated that, like the suspended stone, it will acquire motion if left free, it is said to have energy of position, or potential energy. The monkey of a piledriver while held suspended aloft, the weight of a clock. when wound up, a bent bow, have only potential energy. The monkey descending on the head of the pile, a heavy fly-wheel in rapid motion, a cannon ball flying through the air, are examples of kinetic energy. 42. Kinetic Energy converted into Heat.-We have seen that the kinetic energy of an ascending stone has not perished when the upward motion ceases; it has taken the shape of potential energy, and is recovered without loss as the stone descends. But when the stone is arrested in its descent by the ground, what becomes of its energy then? Even then it is not annihilated. A body that dashes against the ground is found to be sensibly warmer than before. Thus heat is the form that the energy of the moving stone takes when arrested. 43. Heat is Motion.-It is now the universally received doctrine that heat consists of some kind of agitation or vibration of the molecules of matter, and that when we increase the heat of a body, we increase the force of those vibrations. When a moving mass, then, is arrested, we may conceive that the shock makes the molecules of the mass oscillate more widely and rapidly than before; the moving energy of the mass has become molecular energy. 44. Molecular Energy can do Work, as well as the energy of visible motion. For a heated body in contact with a colder communicates part of its heat to the latter —namely, puts its molecules into more rapid agitation. It may even be converted into visible energy; for, by applying the heated body to a portion of colder air, the air will sensibly expand, and may be made to move the piston of a cylinder, and do visible, mechanical work. 45. Energy of Chemical Affinity.-Chemical affinity is a copious source of molecular energy, often of an intense kind. Whenever different atoms unite to form |