Let a card with a coin placed upon it be balanced on one of the fingers of the left hand; then snap it suddenly with the middle finger of the right hand, as represented in Fig. 3. If struck evenly and carefully the card will fly away, leaving the coin balanced upon the finger. In this experiment the inertia of the coin is not overcome by the slight friction of the card, and it therefore remains nearly where it was first placed on the finger. Fig. 3. 16. Porosity is that property of a body by which spaces exist between its molecules. All bodies are more or less porous. Actual cavities or cells that are visible are called Sensible Pores. The invisible spaces that separate all the molecules of a body are called Physical Pores. The metals, in which no pores can be seen even by the aid of the most powerful microscope, are shown to be porous by the fact that by great pressure liquids and gases may be made to pass through them. 17. Divisibility.-All bodies are capable of being divided and subdivided; and in many cases the parts that may be obtained are of almost inconceivable minuteness. The following examples serve to show the extreme smallness of the molecules of matter. A single grain of carmine imparts a sensible color to a gallon of water; this gallon of water may be separated into a million of drops, and if we suppose each drop to contain ten particles of carmine, which is a low estimate, we shall have divided the grain of carmine into ten millions of molecules, each of which is visible to the naked eye. The microscope reveals to us, in certain vegetable infusions, animalcules so small that several hundreds of them can swim in a drop of water that adheres to the point of a needle. These little animals are capable of motion, and even of preying upon each other; they therefore possess organs of motion, digestion, and the like. How minute, then, must be the molecules which go to make up these orgaus! A grain of musk is capable of diffusing its odor through an apartment for years, with scarcely an appreciable diminution of its weight. This shows that the molecules of musk continually given off to replenish the odor are of inconceivable smallness. The blood of animals consists of minute red globules swimming in a serous fluid; these globules are so small that a drop of human blood no larger than the head of a small pin contains at least 50,000 of them. In many animals these globules are still smaller; in the musk deer, for example, a single drop of blood of the size of a pin's head contains at least a million of them. 18. Compressibility is the property of being reduced to a smaller space by pressure. This property is a consequence of porosity, and the change of bulk comes from the particles being brought nearer together by the pressure. Sponge, india-rubber, cork, and elder-pith are examples of compressible bodies; they may be sensibly diminished in volume by the pressure of the fingers. Gases are, however, the best examples of compressible bodies. Some of the gases may be reduced to liquids by pressure alone; and recent experiments have proved that all the gases known can be liquefied by great pressure and intense cold combined. Liquids are but slightly compressible; but careful experiments have shown that they yield somewhat to great pressure. Metals are compressible, as is shown in the process of stamping coins, medals, and the like. 19. Expansibility is the property which a body possesses of increasing in bulk or volume under certain circumstances. All bodies expand on being heated. Gases expand most, liquids next, and solids least, when subjected to the same degree of heat. The molecules of air and the gases constantly repel each other, so that these substances have a continual tendency to increase in volume, even without the influence of heat. The following experiment illustrates this property of air. A small rubber bag, nearly empty and fastened at the neck with a stop-cock, is placed under the receiver of an air-pump. Then let the air be pumped out from the receiver, so that it no longer exerts pressure on the outside of the bag, and the air within will expand and fully inflate the bag. Other examples of expansibility will be given hereafter in illustrating the effects of heat. 20. Elasticity is the property which bodies possess of recovering their original shape and size after having been either compressed or extended. Fig. 4. Bodies differ in their degree of elasticity, yet all are more or less elastic. India-rubber, ivory, and whalebone are examples of highly elastic bodies. Putty and clay are examples of those which are only slightly elastic. If air be compressed, its elasticity tends to restore it to its original bulk; this property has been utilized in making air-beds, air-cushions, and even in forming car-springs. If a spring of steel be bent, its elasticity tends to unbend it; this principle is employed in giving motion to watches, clocks, and the like. If a body be twisted, its elasticity tends to untwist it, as is observed in the tendency of yarn and thread to untwist; this principle, under the name of torsion, is used to measure the deflective force of magnetism. If a body be stretched, its elasticity tends to reduce it to its original length, as is shown by stretching a piece of india-rubber, and then allowing it to contract. We see that the elasticity of a body may be brought into play by four different methods: by pressure, by flexure or bending, by torsion or twisting, and by tension or stretching. In whatever way it may be developed, it is the result of molecular displacement. Thus, when air is compressed, the repulsions between the molecules tend to expand it. Again, when a spring is bent, the particles on the outside are drawn asunder, whilst those on the inside are pressed together; the attractions of the former and the repulsions of the latter tend to restore the spring to its original shape. The most elastic bodies are gases; after them come tempered steel, whalebone, india-rubber, ivory, glass, etc. Fig. 5 illustrates the method of showing that ivory is elastic, and at the same time that the cause of its elasticity is molecular displacement. It represents a polished plate of marble, over which is spread a thin layer of oil. If a ball of ivory be let fall upon it from different heights, it will at each time rebound, leaving a circular impression on the plate, which is the larger as the ball falls from a greater height. This experiment shows that the ball is flattened each time by the fall, that the flattening increases as the height increases, and that the repellent action of the compressed molecules causes it to rebound. Fig. 5. The property of elasticity is utilized in the arts in a great variety of ways. When a cork is forced into the mouth of a bottle, its elasticity causes it to expand and fill the neck so as to render it both water and air tight. It is the elasticity of air that causes indiarubber balls, filled with air, to rebound when thrown upon hard substances. It is the elasticity of steel that renders it of use in springs for moving machinery, as well as for easing the motion of carriages over rough roads. It is the elasticity of cords that renders them suitable for musical instruments. It is the elasticity of air that renders it a fit vehicle for transmitting sound. SECTION II. SPECIFIC PROPERTIES OF MATTER. 21. The specific or characteristic properties of matter depend upon certain forces, which are continually acting between the molecules of bodies. Those which cause the molecules to attract one another are called Molecular Forces. They are Cohesion, Adhesion, and (Chemical fert Affinity.) These act only at insensible distances. The ultimate particles, even of solid bodies, do not touch one another, but are kept in place by the combined action of forces of attraction and repulsion. Heat is the repellent force that tends to separate the molecules; although not usually classed as a molecular force, it here acts as one, and, like those mentioned, at insensible distances. Chemical affinity belongs to Chemistry, and will not be considered here. 22. Cohesion and Adhesion. COHESION is the force двига that holds molecules of the same kind together. If a solid body be broken or divided in any way, the parts cannot, in general, be made to cohere by simply bringing them together. |