CHAPTER III

ENERGY, HEAT, AND PRESSURE

Definition of Hypothesis, Theory, Law-What is Meant by Energy-Effects of Heat-Different Ways in which Heat is Transmitted-Difference between Amount and Degree of Heat-Heat Units-Specific Heat-Latent Heat-Heat of Fusion-Relation of Pressure and Heat-Atmospheric Pressure The Effect of Pressure upon the Boiling Point of Liquids-The Effect of the Specific Gravity of Liquids upon their Boiling Point.

THE changes that occur in nature, either chemical or physical, are not carried on in a haphazard manner, but are controlled by absolute, often unalterable, forces. In referring to or describing these forces of nature, the scientist often speaks of them as laws. There are, in fact, three words often used by scientists in this connection, viz., hypothesis, theory, law.

Definitions of above terms.-An hypothesis is sometimes defined as a suggestion, the truth of which has not been proved, that is advanced as a reason for, or as a cause of, some fact or law. An hypothesis is also often spoken of as a working theory, because an unproved suggestion or idea of what may be the cause for certain effects-in other words an hypothesis-is often the first step in the work of discovering hidden forces of nature or of the application of these forces to practical inventions. By theory is

meant a long-standing hypothesis for phenomena that accounts for them so satisfactorily that it seems to be true, though its validity has never been proved. When a theory has been tested in every known way and is proven to be invariably accurate it is called a law.

The following laws and hypotheses, which are a few of those frequently referred to in physics and chemistry, will be often alluded to in this or following chapters.

Atomic hypotheses.-(1) All elements are made up of minute, independent particles called atoms. (2) All atoms of the same kind of element are alike, atoms of different kinds of elements are different. (3) All atoms combine by wholes to form compounds; atoms are never divided in chemical reactions.

Law of definite composition.-The composition of chemical compounds is not altered by changes in their physical state; e. g., water, whether it exists in liquid, solid, or gaseous form, consists of H2O.

Law of the conservation of energy.-Energy can be changed from one form to another, but it cannot be either created or destroyed. For example, radiant energy from the sun produces chemical energy in plants which causes them to grow. This energy, which binds the atoms of the plant constituents together, remains locked in the molecules and is known as latent or potential energy, but, when the plants are used for food, the energy is liberated from as much of their substance as is oxidized in the body tissues and appears in the form of heat energy and mechanical energy. Or, for another example, the plants that have decayed and in the course of time turned to coal, contain, stored in their substance, the energy, derived from the sun, which furthered their

growth, and when the coal is set on fire this stored energy is transformed into heat, light, or electrical energy; and though energy thus used may seem to have come to an end, it has simply assumed some other form and exists somewhere. Thus the amount of energy in the universe remains unchanged.

Law of the conservation of matter.-Matter can be changed from one form to another, but it cannot be either created or destroyed. For example, a house may be burned, but the elements composing it are still in existence, some in the form of ashes, some as gases that have passed into space. The oxygen of the air, which maintained the combustion, united with some of the carbon and hydrogen in the burning matter, forming thereby carbon dioxid (CO2) and water (H2O); this will be absorbed by plants and by them changed to the substances they need for their growth, and while doing this they will give off oxygen and thus the oxygen will be returned to the air once If the plants die, the elements of which they are composed may be scattered, but they will still exist.

more.

Laws of boiling.—(1) Under a given pressure, every liquid has a definite boiling point; for example water boils at 100° C., alcohol boils at 80° C., mercury boils at 350° C. (2) When the boiling point is reached the temperature remains constant unless the pressure is increased.

Law of Boyle.-The volume of a gas varies inversely with the pressure that is put upon it; i. e., when a gas is under heavy pressure, it will occupy but a small space, but when pressure is released, the gas will expand and thus increase its volume.

Law of Charles.-Every true gas expands 23 of its

volume for each degree centigrade that its temperature is increased and it contracts of its volume for each degree centigrade that its temperature falls.

Laws of liquid pressure.-(1) Pressure in liquids is proportional to the depth alone and is not influenced by the size or shape of the vessels which contain them. (2) At any given depth the pressure is equal in all directions.

Laws of pressure in gases.-(1) Pressure in gases increases with depth, but is not proportional to it. (2) At any given depth the pressure is equal in all directions.

Difference between the use of the terms gas and vapor. The word gas is usually applied to matter like nitrogen, oxygen, and hydrogen which can be liquefied only under high pressure, and the term vapor is used to express the gaseous state of matter that is liquid under ordinary conditions, e. g., water, alcohol, ether. The term aqueous vapor is often applied to vapor derived from water. Vapors and gases are governed by the same laws.

Energy

By energy is meant the power to produce motion, or, in other words, the power to perform work. Energy manifests itself in several forms,-for example, we have heat, light, mechanical energy, electrical energy, and chemical energy, and one form can be made to produce another form, or, in the words of the law of the conservation of energy, energy can be changed from one form to another. Heat, for example, or chemical action can be made to produce an electric current and an electric current will produce heat and promote chemical activity in matter.

When a body is at work its energy is spoken of as kinetic energy (from the Greek kinema-motion). When a substance containing the power of work within itself or of causing motion in other matter is apparently at rest, its power is spoken of as latent or potential energy.

A body possesses energy only because it has received energy in some form and it possesses only as much as it has received. Nearly all the available energy upon earth is derived primarily from the sun. The most obvious example of the sun as a source of energy is that given under the law of the conservation of energy. As there stated, the sun provides the energy which causes plants to grow, and plants pass this energy on to man and other animals who use plants as food, or when substances derived from plants, as wood, coal, etc., are burned the energy they liberate is used to run machinery, produce electric currents or perform other work. Other examples of the sun as a source of energy will be seen in the sections on radiation and reflection.

Heat

As can be seen in the laws and hypotheses on page 39, heat and pressure are two important factors in causing changes in matter.

Definition. Physics and physiology give somewhat different definitions for the term heat. A definition given in physics is the molecular motion of a body, for heat is due to the vibration of the molecules of matter, and the greater the degree of vibration, the more intense the heat, and, conversely, the more intense the heat, the more rapid the motion. It is because heat increases molecular motion that it