CHAPTER II THE PROPERTIES OF MATTER 9. General and Specific Properties. In considering the properties of matter a distinction should be made between those properties that belong to matter itself and those that belong to bodies only. General properties are those found in all matter, such as extension, division, impenetrability, porosity, inertia. Specific properties are those found in certain kinds of matter only, such as ductility, hardness, malleability. FIG. 1 I. GENERAL PROPERTIES 10. Impenetrability. Space that is occupied by one portion of matter cannot at the same time be occupied by any other portion. This is a property of matter rather than of bodies. Demonstration. - Push the closed end of a test tube into the water in a graduate that is partly full, as shown in Fig. 1. The difference between the readings of the water surface before and after the tube is inserted will give the volume of the submerged part of the tube, which is the same as that of the displaced water. A nail driven into a block of wood pierces the block and pushes the substance of the wood together. The block is usually not increased in size, but the wood now occupies only part of the space it originally occupied. 11. Porosity. A body is said to be porous, or to have porosity, because the particles of matter of which it is composed do not fill the entire volume occupied by it. The pores in bodies vary in size from those that can be seen in a sponge or a piece of charcoal, to those in stone or metal, which may be invisible even though a microscope of the highest power be used. All bodies are porous. The fact that a blotter will absorb ink and that a drop of oil will pass into a fine-grained piece of polished marble depends upon the porosity of the blotter and the marble. Demonstrations. - The porosity of leather may be proved by the use of the apparatus shown in Fig. 2. The funnel A has a long stem over the end of which there is securely tied a piece of stretched wash leather B. Pour mercury into the funnel, and it will pass through the pores in the leather and fall in the form of a fine rain into the jar below. As the mercury is thus freed from all mechanical impurities, the process is of practical value. Pour water into a glass tube about a meter long, until it is nearly half full, and then add colored alcohol until the surface is a half inch from the end of the tube. If this is done carefully, the line of division between the water and the alcohol can be clearly seen. Now place the finger over the end of the tube and invert it. The water will be seen to flow down through the lighter alcohol, and minute bubbles of air will rise through the mixed liquid. Invert the tube two or three times, and the length of the liquid column will be half an inch or more less than before. What does this demonstration teach? FIG. 2 The existence of spaces that are not occupied by the molecules of a liquid can be illustrated by filling three glasses, one with smooth round peas, one with fine shot, and one with water. Let the first be level full, then pour shot upon the peas and shake down, being careful that the surface of the peas is not raised. When no more shot can be put in, pour in water until it comes to the top of the peas. 12. Compressibility. Since the volume of a body can be reduced by pressure, bodies are said to be compressible. This property depends upon porosity. Gases are very compressible, solids to a much less degree, and liquids are almost incompressible. By doubling the pressure upon a gas its volume is diminished one half, while changing the pressure upon water from 15 lb. per square inch to 30 lb. per square inch diminishes its volume only 2000. 13. Indestructibility. Matter can be made to assume different forms as the result of physical changes, and it can be combined with other matter, or broken up into different kinds of matter, through chemical forces; but matter itself cannot be destroyed. The disappearance of visible matter in the boiling away of water is only a change from the liquid to the vaporous condition. If, after burning a piece of coal, all the products of the combustion (both solids and gases) are carefully weighed, it is found that the sum of their weights is the same as the weight of the coal and the oxygen used up in the combustion. 14. Divisibility. Bodies can be divided into smaller parts without changing the matter composing them. Divisibility by mechanical means has practically no limit. The finest crayon dust is made up of small bodies of chalk, as may be seen by examining it with a microscope. Demonstration. - Drop a little red ink upon the surface of water in a beaker. It will mix gradually, and a thread of colored water will pass slowly downward. Let the beaker stand undisturbed for a few days, and the ink will be distributed uniformly throughout the water. Its finely divided state is shown by the fact that every drop of the solution is visibly colored. 15. Inertia is the tendency a body has to retain its condition of rest or motion. Whenever a body is at rest, it can be put in motion only by some force outside of itself; and whenever a body is in motion, the rate or direction of this motion can be changed only by the application of a force from without the body. This property is, therefore, purely negative. Demonstration. - Place a smooth flat card on the mouth of a bottle with a small neck, and on it put a small marble exactly over the mouth of the bottle. A snap with the finger on one corner of the card will send it spinning across the room, while the ball will drop into the bottle. Place a little cotton inside the bottle. Why? Inertia is illustrated in a great many accidents: for example, the spilling of a liquid in a dish that is moved too quickly; the shock to a railway passenger when the air brakes are applied too suddenly; the fall caused by jumping from a rapidly moving car. Use is made of inertia, as in driving on the head of a mallet by striking the end of the handle; throwing an apple stuck on the end of a rod; or changing the position of a column of mercury in a glass tube by jerking the tube lengthwise. 16. Elasticity. When a tennis ball is compressed into smaller volume between the hands, it feels springy, or elastic, because the contained air tends to resume its original volume; that is, the molecules tend to regain their original distance apart. All gases and liquids, and to a certain ex tent solids also, show a similar reaction against external pressure, and are said to have elasticity of volume. When the shape of a solid body is slightly changed by an external force, it usually tends to resume its original form. This elasticity of form may be classified as follows (the corresponding external forces being given in parentheses): (a) Elasticity of Compression (pressure); it is shown, for example, by the rebound of a solid rubber ball dropped upon the floor, or by the recoil of a compressed spring. (b) Elasticity of Traction (pulling); it is illustrated by a rubber band stretched around a book and holding it shut. (c) Elasticity of Flexion (bending); it is shown by the vibration of a tuning fork, or of a steel wire one end of which is clamped in a vise. (d) Elasticity of Torsion (twisting); an example is the untwisting of a rubber tube when it is held at one end and the other let go after being twisted. 17. Hooke's Law; Elastic Limit. If different weights are suspended from the hook of a spring balance, it will be found that the stretch of the spring for a three-pound weight is three times as great as for a one-pound weight, and that the stretch for six pounds is twice as great as for three. Further experiment will prove that up to a certain point the elongations of any spiral spring are proportional to the weights used, and that the spring returns exactly to its original length as soon as the weight is removed. But if too great a weight is put upon the spring, a permanent change in its shape is brought about and the pull is said to have exceeded the elastic limit of the spring. The above experiment illustrates only one case of a law stated by Robert Hooke in 1676 and now known as Hooke's Law. This is |