but a molecule of common salt consists of an atom of the metal sodium combined with an atom of chlorine.

Atoms are joined and held together by a kind of attraction called

chemical affinity.

Molecules are kept in place by the action of two opposing forces, molecular attraction and molecular repulsion.

4. Mass and Density. - The MASS of a body is the quantity of matter which it contains.

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

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. The masses of bodies are proportional to their weights.

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.

5. Three States of Bodies.. Bodies may exist in three different states, solid, liquid, and aeriform.

SOLID bodies tend to retain a permanent form, because their molecules are held together by forces of attraction which are greater than the repellent forces that would tend to separate them.

In LIQUIDS the attractive and repellent forces are nearly balanced, and their molecules move freely among one another. Liquids have no tendency to retain a permanent form, but assume at once the form of the containing vessel.

In AERIFORM bodies the repellent are more powerful than the attractive forces, and their molecules constantly tend to separate and occupy a greater space. Air and all gases and vapors are examples of aeriform bodies.

The term FLUID is applied to both liquid and aeriform bodies. Many bodies may exist in every one of the three states in succession. Thus, if ice be heated until the repellent forces balance those of at

traction, 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.

6. The most important properties which all bodies possess are: Extension, Weight, Impenetrability, Inertia, Porosity, Divisibility, Compressibility, Expansibility, and Elasticity.

7. Extension is the property by virtue of which a body occupies space.

MAGNITUDE and FORM depend upon Extension.

To occupy space a body must have the three dimensions, length, breadth, and thickness. The space occupied by a body is called its volume.

8. English Measures. For the purpose of measuring the dimensions of bodies a standard unit of length is needed.

In England and the United States the yard has been adopted as the standard unit, and with its divisions and multiples is in commou

use.

9. The Metric System. This system is in general use in France and in most of the countries of Europe.

It is adopted by scientific writers everywhere, and will probably soon come into general use throughout the civilized world. Its use in the United States has been legalized by act of Congress.

The unit of this system is the meter, which is the ten-millionth part of a quadrant of that meridian of the earth which passes through Paris. It is equal to 39.37 inches, nearly.

Its divisions and multiples vary in a tenfold ratio, and from these all the measures of surface, volume, and weight are derived. The nomenclature is derived from the Greek and Latin numerals. The Greek prefixes deka (10), hekto (100), kilo (1000), and myria (10,000), are used for the multiples, and the Latin prefixes deci (1), centi (1), milli (1), are used for the divisions of the unit.

10. Metric Measures of Length. In the following table the several denominations of linear measure are given

in their order, with the English equivalents, and the abbreviations used.

Fig. 1.

2

In the figure in the margin one decimeter is compared with a scale of inches. It will be seen that the decimeter is a little less than 4 inches.

With the square meter and the cubic meter as units, tables are constructed for the measures of surface and volume, in the same way as with the English measures; the ratio 100 (102) being used for surface measures, and 1000 (103), for volumes.

Thus, 100 square millimeters = 1 square centimeter, etc.; 1000 cubic millimeters = 1 cubic centimeter, ete

11. Measures of Capacity. - For measuring articles which by the English system are sold by dry or liquid measure, the unit adopted is the liter, which is equal to one cubic decimeter.

The denominations are as follows. Ratio 10.

1

2

3

4

5

7

S

9

10

It

The liter is equal to 1.0567 liquid quarts, or 0.908 of a dry quart. may therefore be used conveniently in place of both.

12. Weight. A body falls, when not supported, because it is attracted toward the centre of the earth. When it rests upon another body or upon the surface of the earth, its tendency to fall is not destroyed, and it presses downward with a force proportioned to the degree in which it is attracted. Hence weight is the measure of the earth's attraction. The term weight is commonly used in this limited sense, but, since the attraction of gravitation is universal, a body would have weight if placed on or near the surface of any of the planets or other heavenly bodies.

The unit of weight in the English system is the avoirdupois pound of 7000 grains. In the Metric System the unit adopted is the gram, which is the weight of one cubic centimeter of distilled water at its greatest density, that is, at the temperature of 39.2° Fahrenheit or 4° Centigrade.

13. Metric Table of Weight.

Ratio 10. 0.0154 grain

14. 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 diminution of volume arises from the particles of one of the fluids insinuating themselves between those of the other; but it is clear

that where a particle of alcohol is, there a particle of water cannot be.

It may be shown by several simple experiments that air and water cannot occupy the same space. Invert a glass tumbler and press it downward into a vessel of water. The water will not enter and fill the tumbler. Close one end of a glass tube with the thumb and thrust the other end into the water. The water cannot fill the tube while the air is retained. Remove the thumb so that the air can escape, and the water will immediately rise and fill the tube. Pass a funnel through a cork fitted air-tight to a bottle. Let a bent tube pass through another hole in the cork, and at the other end dip into a tumbler of water, as shown in Fig. 2. If then water is poured into the funnel, as fast as it enters the bottle air will escape in bubbles from the end of the tube in the tumbler.

Fig. 2.

15. Inertia is the tendency which a body has to maintain its state of rest or motion. If a body is at rest it has no power to set itself in motion, or if it is in motion it has no power to change either its rate of motion or the direction in which it is moving. Hence, if a body is at rest, it will remain at rest, or if in motion, it will move on uniformly in a straight line until acted upon by some force.

The reason why we do not see bodies continue to move on uniformly in straight lines, when set in motion, is that they are continually acted upon by forces which change their state of motion. Thus, a ball thrown from the hand, besides meeting with the resistance of the air, is continually drawn downwards by the attraction of the earth, till at last it is brought to rest.

Many familiar phenomena are explained by the principle of inertia. For example, when a vehicle in motion is suddenly arrested, loose articles in it are thrown to the front, because they tend to keep the motion which they had acquired.

If a person jumps from a car in rapid motion, he is likely to be thrown violently to the ground; for his body retains its onward motion, while his feet are stopped by striking the ground.