Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

Impressions and ideas rest which constituted Hume."

4. Origin of the word Spirit.—The name spirit came originally from an attenuated form of matter, the air or breath, because the air, like spiritual existence, is invisible. The formation of language is to a large extent thus based on analogies.

5. Spirit not an Object of Sense.—None of the senses can perceive spirit itself, though they perceive the effects which spirit produces on material substances. If, for example, you move your arm, it is the spirit within you, acting upon the muscles through the nerves, that causes it to move; and you see here the effects produced by spirit upon matter, but you do not see the spirit itself.

6. Effects of Matter on the Senses.—Some forms of matter can be perceived by all the senses; others can be perceived by only a part of them; some by only one. Air you can not see, nor smell, nor taste; but you can feel it, and hear the sound of its motion. Sometimes matter affects only the sense of smell, or that with the sense of taste. Sea-air smells salt; but the salt in the air is so finely divided that we can not see it. And yet it is the salt, entering the nostrils and coming in contact with the extreme fibres of the nerve of smell, that produces the effect. So when we smell a flower, matter comes from it in particles so fine that no microscope can detect them, but they produce sensation when they strike upon the nerve.

7. Forms of Matter.—Matter appears in three forms: solid, liquid, and gaseous or aeriform—that is, like air. Sometimes matter is spoken of as having only two forms—solid and fluid. In this case fluids are divided into two classes, the elastic and non-elastic. The air and the various gases and vapors are the elastic fluids; while those which are called liquids are the non-elastic fluids. A foot-ball bounds because the air in it is an elastic fluid. If it were filled with a non-elastic fluid; as water, it would not bound. When water takes the form of steam it is an elastic fluid. Though it is very common to use the expression elastic fluids, the division of matter into three forms is the one usually recognized.

8. Solids.—In solid matter the particles can not be moved about among each other; but each particle generally retains the same position in relation to those particles which are around it—in other words, it does not change its neighborhood. This is more true of some solids than of others. It is absolutely true of such hard solids as granite and the diamond. In these the particles are always in the same relative position. But it is not so with gold or lead. By hammering these you can change greatly the relative position of their particles. India-rubber is a solid, but the relative position of its particles can be much altered in various ways.

9. Liquids.—It is the grand characteristic of a liquid that its particles change their relative position from the slightest causes. It is in strong contrast with solids in this respect. When you move any portion of a solid body you move all the other portions of it, and generally in the same direction. But a body of liquid can not be moved all together as one body except by confining it; as, for example, in the case of a water-pipe or a syringe. And then, the moment that the water can escape, the particles use their liberty of altering their relative position. As wind and other agents act continually upon water, no particle stays for any length of time in the neighborhood of the same particles. "Unstable as water" is, then, an exceedingly significant expression. Water is never at rest. A particle of it may at one time be floating on the surface of the ocean, and at another be in depths beyond the sounding of man. It flies on the wings of the wind, falls in the rain, runs in the stream, is exhaled from a leaf, trembles in the dew-drop, flows in the blood of an animal or in the sap of a plant, and is always ready to be jostled along in its ever-changing course.

10. Gases.—The particles of gaseous or aeriform substances move among each other even more freely than those of a liquid. Air, therefore, is more unstable and restless than water. Even when the air seems to be perfectly still its particles are moving about among each other. You can see this to be true if you darken a room, leaving a single shutter a little open. Where the light enters you will see motes flying about in every direction, which would not be the case if the air were really at rest. The particles of air have a greater range of travel than those of water; for the sea of atmosphere which envelops the earth rises to the height of about fifty miles. How far water rises in its evaporation we know not; but it is not at all probable that it rises to the uppermost regions of the atmosphere.

11. Filling of Spaces by Liquids and Gases.—It is the freeness with which the particles of liquids and gases move among each other that enables them to insinuate themselves into spaces every where. They are ever ready to enter into any substances which have interstices or pores of such size as will admit them. There are mingled with the grains of the soil not only water, but air and gases. These are present also in all living substances, both vegetable and animal. Water is the chief part of sap and of blood, and air and gases always go with water. Part of the air that we breathe in enters the blood in the lungs, and courses with it through the system. The fishes could not live in water if there were not air mingled with it. This can be proved by experiment. If you put a fish into a close vessel it will soon die, because it uses up all the air that is in the water. In an open vessel the fish is kept alive by the constant accessions of fresh air to the water.

12. Solution.—In solutions of solid substances in water it is the freedom with which the particles of water move about among each other that enables them to take in among them the minute particles of the solid. And when water ascends into air by evaporation it may be said to be a real solution of water in the air; for the particles of water mingle with those of the air, just as the particles of a solid mingle with those of water in a solution.