This book is written in the hope that it will make more scientific the student's common knowledge of the forces and changes in the world about him and will give him many ideas and principles that will help him to acquire the habit of looking from effects to their natural causes and thus tend to develop what is called the _scientific habit of thought_. =3. Hypothesis, Theory, and Law.=--Three words that are frequently used in science may be mentioned here: _hypothesis_, _theory_, and _law_. An hypothesis is a supposition advanced to explain some effect, change, or condition that has been observed. For example, the Nebular Hypothesis of which many high-school students have heard, is an attempt to explain the origin of the sun, the earth, the planets, and other solar systems. A theory is an hypothesis which has been tested in a variety of ways and which seems to fit the conditions and results so that it is generally accepted as giving a satisfactory explanation of the matter in question. The Molecular Theory of Matter which states that matter of all kinds is composed of very small particles called molecules (see Art. 6), is a familiar example of a theory. A theory becomes a law when it may be definitely proved. Many laws are expressed in mathematical language, e.g., the law of gravitation. (See Art. 88.) Many of the laws of physics are illustrated by laboratory experiments, which show in a simple way just what the law means. Exercises Explain what is meant by the following terms and expressions: 1. Common knowledge. 2. Scientific knowledge. 3. Science. 4. Topics in physics. 5. Scientific habit of thought. 6. Value of relating new ideas to former experiences. 7. Hypothesis. 8. Theory. 9. Law. (2) THE STATES OF MATTER =4. Physics Defined.=--In the study of any science or field of knowledge, it is helpful to have a basis for grouping or classifying the facts studied. In physics we are to study the objects, forces, and changes about us, to understand them and their relations to one another. Accordingly, physics, dealing with the material world about us, is often defined as _the science of matter and energy_, _matter_ being _anything that occupies space_ and _energy_ the _capacity for doing work_. This definition of physics while not strictly accurate is sufficiently comprehensive for our present purpose. =5. The Three States of Matter.=--Our bodies are _matter_ since they occupy space. Further, they possess _energy_ since they are able to do work. In beginning the study of physics it will simplify our work if we study one of these topics before the other. We will therefore begin with matter and consider first its three states.