## 1. Composition Of Matter

Composition Of Matter. Matter-that is, the substance of which bodies are composed-is made up of molecules. A molecule is defined as the smallest portion of matter that can exist in an independent state. Every body-is composed of millions of these molecules, held together by a force called cohesion. It is supposed that these molecules are separated by spaces, and that they are continually in a state of motion, vibrating back and forth, with a greater or less velocity, and continually approaching and receding from one another.

## 2. Heat A Form Of Energy

Heat A Form Of Energy. The motion of the molecules composing matter gives rise to the phenomenon of heat, and causes the sensation of warmth or cold. When the motion is slow, the body feels cold to the touch; if, on the other hand, the vibrations are rapid, the body is warm or hot. Heat is therefore not a substance, but a manifestation of motion, or, as it is usually expressed, heat is a form of energy.

## 3. Temperature

Temperature. Temperature is a term used to indicate how hot or cold a body is-i. e., to indicate the rate, of vibration of the molecules of a body. A hot body has a high temperature; a cold body, a low temperature. When a body, as, for example, an iron bar, receives heat from any source, its temperature rises; on the other hand, when a body loses heat, its temperature falls.

The temperature is not a measure of the quantity of heat a body possesses. Temperature may be considered to be a measure of the velocity of the molecules of a body as they vibrate to and fro, while the quantity of heat may be considered to be the kinetic energy of the molecules composing the body. A small iron rod may be heated to whiteness and yet possess a very small quantity of heat. Its temperature is very high, which simply indicates that the molecules of the rod are vibrating with an extremely high velocity.

Temperature is measured by an instrument called a thermometer, which is so familiar as to scarcely need description. It consists of a thin glass tube, at one end of which is a bulb filled with mercury. Upon being heated, the mercury expands in proportion to its temperature. Thermometers are graduated in different ways. In the Fahrenheit thermometer, which is generally used in this country, the point where the mercury stands when the instrument is placed in melting ice is 32°. The point indicated by the mercury when the thermometer is placed in water boiling in open air at the level of the sea is marked 212°. The tube between these two points is divided into 180 equal parts, called degrees.

## 4. Latent Heat

Latent Heat. Suppose we take a block of ice at a temperature of, say, 14° and heat it. If a thermometer is placed in contact with the ice, its temperature will rise until it reaches'32°, and will then remain stationary. As soon as this temperature is reached, the ice begins to melt, or change to water, and the heat, instead of raising the temperature further, is all used to effect this change of state. In a similar manner, if sufficient heat be applied to a quantity of water, it will eventually change it to steam. One of the effects of heat, therefore, is the changing of a solid to a liquid, or of a liquid to a gas. The heat which is thus expended in changing a body from the solid to the liquid state, or from the liquid to the gaseous state, is called latent heat. The portion of the heat applied which raises temperature, and which, therefore, affects the thermometer, is sometimes called sensible heat.

## 5. Measurement Of Heat

Measurement Of Heat. Since heat is not a substance, it cannot be measured directly in pounds or quarts; but, like force, it may be measured by the effects it produces. Suppose a certain quantity of heat raises the temperature of a pound of water from 52° to 53°. It will take the same quantity of heat to raise the pound from 53° to 54°, and it will take double the quantity to raise the temperature of the pound of water from 52° to 54° that it took to raise the temperature from 52° to 53°. The unit quantity of heat is the quantity required to raise the temperature of a pound of water from 62° to 63°. This unit is called the British thermal unit, or B. T. U.