POPULAR PHYSICS.

INTRODUCTION.

CLASSIFICATION OF THE SCIENCES.

SCIENCE is a knowledge of the laws that govern the Universe.

A Law is a necessary relation between cause and effect. It is assumed as the foundation of all Science, that like causes produce like effects. This principle is an inductive truth, founded upon universal experience.

By the UNIVERSE we mean all that has been created, whether material or immaterial. The Universe may be regarded as made up of mind and matter. MIND is that which thinks and wills; MATTER is that of which we become cognizant through the medium of the senses. Science admits of two corresponding divisions, Science of Mind, or METAPHYSICS, and Science of Matter, or NATURAL PHI

LOSOPHY.

NATURAL PHILOSOPHY is that branch of science which treats of the laws that govern the material Universe.

Matter exists in two states, organized and unorganized; it is organized when its particles are aggregated into organs adapted to the support of life; in all other cases it is un

What is Science? What is a Law? Define the Universe. Mind. Matter. What are the two divisions of Science? What is Natural Philosophy? In what two states may Matter exist? Illustrate.

organized. Natural Philosophy admits of two corresponding divisions: Science of Organized Matter, or PHYSIOLOGY, and Science of Unorganized Matter, or GENERAL PHYSICS.

Physiology, which treats of the laws of matter as modified by the principle of vitality, is divided into two principal branches: Animal Physiology, or ZoöLOGY, and Vegetable Physiology, or BOTANY. Both of these branches, with their various subdivisions, belong to the domain of NATURAL HISTORY.

All unorganized matter may be divided into two classes, Celestial and Terrestrial. General Physics admits of two corresponding divisions. That branch which treats of celestial bodies, including the earth as a whole, is called AsTRONOMY; that which treats of terrestrial bodies, is called TERRESTRIAL PHYSICS.

TERRESTRIAL PHYSICS is again subdivided into two branches. The first is called Physics Proper, or simply PHYSICS; it treats of the general properties of bodies. The second is called CHEMISTRY; it treats of the nature of the ultimate particles of bodies and of their laws of combination. The first of these branches, or PHYSICS, is the subject treated of in the following pages.

Besides the branches above enumerated, and which may be called Pure Sciences, there are others that depend upon, or are applications of, two or more of them. Such, for example, are the sciences of GEOLOGY, MINERALOGY, PHYSICAL GEOGRAPHY, &c. These are called Mixed Sciences.

Into what may Natural Philosophy be divided? What is Physiology, and what are its branches? How may Unorganized Matter be divided? What are the corresponding divisions of General Physics? Define them. How is Terrestrial Physics divided? What is Physics Proper? Chemistry? What are the Pure Sciences, and what are some of the Mixed Sciences?

CHAPTER I.

PRELIMINARY PRINCIPLES AND MECHANICS OF SOLIDS.

I.-DEFINITIONS AND GENERAL PROPERTIES OF MATTER.

Definition of Physics-Physical Agents.

1. PHYSICS is that branch of Natural Philosophy which treats of the general properties of bodies, and of the causes that modify these properties.

The principal causes that modify the properties of bodies are: Gravitation, Heat, Light, Magnetism, and Electricity. These causes are called Physical Agents.

Definition of a Body.

2. A BODY is a collection of material particles; as a stone, or a block of wood. A body which is exceedingly small is called a Material Point.

Bodies are made up of small particles, called Molecules, and these again are composed of still smaller elements, called Atoms. These atoms are inconceivably small, and are held in their places by the action of two opposing systems of forces, called Molecular Forces. Those which tend to draw atoms together are called Attractive Forces, and those which tend to push them asunder are called Repellent Forces. Heat is the principal if not the only repellent force in Nature.

(1.) What is Physics? What are Physical Agents? Name them. (2.) Define a Body. A Material Point. An Atom. A Molecule. What are Molecular Forces? Define Attractive and Repellent Forces.

Mass and Density.

3. The MASS of a body is the quantity of matter which it contains.

Different bodies, having the same volume, contain very different quantities of matter; for example, a cubic inch of lead contains nearly eleven times as much matter as a cubic inch of water. masses of bodies are proportional to their weights.

The

The DENSITY of a body is the degree of closeness of its particles.

Those bodies in which the particles are close together are said to be dense; thus, platinum and mercury are dense bodies. Those in which the particles are not close together are said to be rare; thus, steam and air are rare bodies. The densities of bodies having the same bulk are proportional to their weights.

4.

Classification of Bodies.

Bodies may exist in two different states, the solid and the fluid.

SOLIDS are those which tend to retain a permanent form; as stones, metals, and the like. The particles of such bodies adhere to each other with considerable energy, and this adhesion can be overcome only by the exertion of some effort.

FLUIDS are those whose particles move freely amongst each other; as water, alcohol, and air. Such bodies have no tendency to retain a permanent form, but assume at once the form of the containing vessel.

Fluids are divided into Liquids and Gases or Vapors, Liquids are sensibly incompressible; as water, wine, and milk. Gases and vapors are highly compressible; as atmospheric air and steam.

(3.) What is the mass of a body? Density? Give examples of dense and rare bodies. (4.) How are bodies divided? Define solids and fluids. How are fluids divided? Define liquids, and gases or vapors.

In solids, the molecular forces of attraction are greater than the repellent forces, hence the difficulty of separating their molecules; in liquids, the attractive and repellent forces are sensibly balanced; in gases, the repellent are more powerful than the attractive forces.

Many bodies may exist in each of the three states in succession. Thus, if ice be heated until the repellent forces balance those of attraction, it passes into the liquid state and becomes water; if still more heat be applied, the repellent forces prevail over those of attraction, and it passes into the state of vapor and becomes steam.

General Properties of Bodies.

5. All bodies possess certain properties, the most important of which are: Magnitude, Form, Impenetrability, Inertia, Porosity, Divisibility, Compressibility, Dilatability, and Elasticity.

Magnitude and Form.

6. The MAGNITUDE of a body is its bulk, or the portion of space that it fills. It is evident that a body can not exist without possessing the three attributes of length, breadth, and thickness.

The FORM of a body is its external shape. Bodies may have the same magnitude and be very different in shape; they may likewise be of the same form and yet be of very different magnitudes.

Impenetrability.

7. IMPENETRABILITY is that property by virtue of which no two bodies can occupy the same place at the same time. This property is self-evident, although phenomena are observed which would seem to conflict with it. Thus, when a pint of alcohol is mixed with a pint of water, the volume of the resulting mixture is less than a quart. This diminu

Illustrate. (5.) What properties belong to all bodies? (6.) What is Magnitude? Form? (7.) What is Impenetrability? Illustrate.