dulum CB, suspended at 0. A weight, A, slides along the rod C, and may be set so as to make the vibrations as slow or as rapid as may be desired. The instrument is set by means of a scale, marked on the rod, so that any number of oscillations may be made in a minute. The pendulum is sustained by a coiled spring which sets in motion a train of wheels, somewhat in the manner of a clock. In the drawing the weight is set at 92, which shows that it is to make 92 oscillations per minute. IV. PRINCIPLES DEPENDENT ON MOLECULAR ACTION. Molecular Forces. 57. BESIDES the forces which act upon bodies from without and at sensible distances, there is another class of forces continually exerted between the molecules of bodies, and acting only at insensible distances. These forces are called Molecular Forces, and are both attractive and repellent. The molecules of bodies are held in equilibrium by these forces, and it is to them that are to be attributed many of the most important physical properties. The ultimate particles of bodies do not touch each other, being kept asunder by a force of repulsion, which we have said is in general due to heat; they are prevented from receding from each other too far by a force of attraction, and it is only when these forces just balance each other throughout the body, that it is in equilibrium. When a body is compressed, the forces of repulsion are called into play, and, acting like coiled springs, they tend to restore the body to its primitive form. In like manner, when a body is elongated, or stretched, the forces of attraction are called into action and tend to restore the body to its primitive form. Describe it. (57.) What are Molecular Forces? How divided? How are molecules held in place? To what is the repellent force due? Explain the effects of compressing and stretching bodies. Cohesion. 58. COHESION is the force of attraction which holds the molecules of the same body together, as, for example, in a mass of iron, or of wood. Cohesion differs from chemical affinity, which determines the molecules by uniting dissimilar atoms according to fixed laws. Chemical affinity unites atoms of carbon, oxygen, and hydrogen, to form molecules of sugar; but it is cohesion that unites the molecules of sugar into a solid body. The strength of bodies depends upon cohesion. When a body offers a strong resistance to forces tending to tear it asunder, it is said to be tenacious; for example, iron or steel wires, and the like, are highly tenacious. Adhesion. 59. ADHESION is the force of attraction which holds the molecules of dissimilar bodies together. Thus, it is adhesion which causes paint and glue to adhere to wood. If two polished bodies are brought into contact, and pressed together, they will adhere with considerable force. If two plates of glass be ground so as to fit closely, and a little oil be interposed, it is very difficult to separate them. If two hemispheres of lead be pressed together, after having their plane surfaces well polished, they will adhere very strongly. It is adhesion which renders it difficult to raise a wooden board from the surface of the water on which it floats. It is also adhesion between the particles of wood and water, that causes water to spread over a piece of wood upon which it is poured. Solution is due to adhesion. Thus, when sugar dissolves in water, it is because the adhesion between the molecules of sugar (58.) What is Cohesion? Example. Difference between cohesion and chemical affinity? Illustrate. When is a body tenacious? (59.) What is Adhesion? Example. Explain adhesion of metallic surfaces. Of board to water. Explain the phenomenon of solution. and water is stronger than the cohesion between the molecules of sugar. If a liquid tends to spread itself over a solid body, it is said to wet it, as water upon glass. If it gathers in globules, it does not wet it, as quicksilver upon glass. Capillary Forces. 60. CAPILLARY FORCES are molecular forces, exerted between the particles of a solid and those of a liquid. They are called capillary, because their effect is mostly observed in capillary tubes, that is, tubes of the diameter of a hair. The following are some of the phenomena of capillarity: 1. When a body is plunged into a liquid which is capable of wetting it, as when a glass rod is plunged into water, it is observed that the liquid is slightly elevated about the body, taking a concave form, as shown in Fig. 40. Fig. 40. Fig. 41. Fig. 42. 2. If a hollow tube is used instead of a rod, the liquid will also rise in the tube, as shown in Fig. 41. The smaller the bore of the tube, the higher will the liquid rise, and the more concave will be its upper surface. (60.) What are Capillary Forces? Why so called? Explain the phenomenon observed when a glass rod is plunged in water. When a tube is plunged into water. 3. When a tube is plunged into a liquid which is not capable of wetting it, as when glass is plunged into quicksilver, the liquid is depressed both on the outside and on the inside, taking a convex surface, as shown in Fig. 42. The smaller the tube, the greater will be the depression, and the more convex will be the upper surface. These capillary phenomena are due to the resultant action of the cohesion of the liquid and the adhesion of the solid and liquid. When the former predominates, the liquid is depressed in the tube. When the latter predominates, the liquid is raised in the tube. Applications of Capillarity. 61. It is in consequence of capillary action that oil is raised through the wicks of lamps, to supply the flame with combustible matter. The fibres of the wicks leave between them a species of capillary tubes, through which the oil rises. If a piece of sugar have its lower end dipped in water, the water will rise through the capillary interstices of the sugar and fill them. This drives out the air and renders the sugar more soluble than when plunged dry into water, in which case the contained air resists the absorption of water, and retards solution. If a bar of lead be bent into the form of a siphon, and the short arm be dipped into a vessel of mercury, the mercury will rise into the lead by capillary action, and flowing over the edge of the vessel will descend along the longer branch and escape from the lower extremity. In this way the vessel may be slowly emptied of the quicksilver. Many fluids may be drawn over the edges of the containing vessels by a siphon of candle-wicking or other capillary substance. When a glass tube is plunged into mercury. Why does oil rise in a wick? Water in sugar? siphon of wicking. Causes of the phenomena. (61.) Absorption. 62. ABSORPTION is the penetration into a porous body, of any foreign body, whether solid, liquid, or gaseous. Carbon, in the form of charcoal, has a great capacity for absorbing gases. If a burning coal be introduced into a bell-glass, filled with carbonic acid, collected over mercury, the volume of the gas is diminished by being absorbed by the coal. It is found that the charcoal absorbs in this way thirty-five times its own volume of the gas. Charcoal also absorbs other gases in even still greater quantities. Spongy platinum absorbs hydrogen so rapidly as to heat the platinum red-hot. In vegetables and animals we have many examples of absorption. The roots of plants absorb from the earth the material necessary to the growth of the stem and branches. In the animal world, absorption plays an important part in the process of nutrition and growth. Animal tissues also absorb solid substances. For example, workmen engaged in handling lead absorb through the skin and lungs more or less of this substance, which often gives rise to very serious diseases. Imbibition. 63. IMBIBITION is the absorption of a liquid by a solid body. Imbibition is an effect of capillarity, for the interstices between the molecules, by communicating with each other, form a mass of capillary tubes, into which the liquid penetrates by virtue of the capillary forces. Such is the cause of wood and earth absorbing water and other liquids. If a damp substance be placed in a dry and porous vessel, it (62.) What is Absorption? Examples. Carbon. Spongy platinum. Vegetables. Animals. (63.) What is Imbibition? What is the cause of imbibition? Examples |