Gravity, cohesion, heat, light, electricity, and magnetism, are the forces chiefly concerned in modifying physical properties, and are therefore known as physical forces.

5. Chemical Properties and Changes. But matter is capable of undergoing changes by which its distinctive characters are destroyed. Thus bright iron, when exposed to damp air, is converted into a brown rust. When vinegar and lime are brought together, they combine, losing their properties, and producing a new and different substance. When wood is heated, in the absence of air, it is changed to a black, brittle mass; if heated in the presence of air, it is changed to invisible gases and ashes. These are examples of the chemical changes of matter.

Chemistry divides all substances into two kinds, simple and compound. Compound bodies are such as can be separated or decomposed into simpler parts; simple bodies, on the contrary, are such as cannot be thus decomposed. Water is a compound, and can be separated into two invisible gases; but neither of these can be again resolved into different kinds of matter; they are, therefore, ranked as simple bodies, or elements. Chemical science treats of the composition of matter, of the nature and properties of its elementary parts, of the compounds which may be. formed from them, and of the laws by which combination and decomposition are governed.

6. Chemical Physics.-No chemical change can occur without being accompanied by some kind of physical change. So intimately are the forces of Nature connected, that the disturbance of any one is certain to involve a variety of effects. Physical forces and conditions have so powerful an influence over chemical actions, that some knowledge of them is indispensable to the chemical student. Accordingly, under the title of "Chemical Physics," we shall first treat briefly of those physical agencies which are most intimately connected with the subject of chemistry.

PART I.

CHEMICAL PHYSICS.

CHAPTER I.

GRAVITY.

§ 1. Absolute Mass, Volume, and Weight.

7. The Measurement of Matter.-The science of chemistry is based upon numerical laws, and the chemist is almost always occupied in investigating quantities, amounts of matter, or amounts of change; and this is done by the operations of weighing and measuring. The amount of any material body occupying space is termed the mass, and the quantity of space so occupied, the volume or bulk of that body. The process by which the volume of any body is determined is termed measurement, and the instruments used for this purpose are called measures of capacity. They consist of vessels of various shapes, always inclosing the same or multiples of the same amounts of space. The units or standard amounts of space to which volumes are referred vary in different countries. For ordinary purposes, gallons, quarts, pints, cubic inches, cubic feet, and cubic yards, are most commonly used with us, but, in making scientific investigations, the metrical scale, also called the

decimal or French scale, of measures is now almost universally employed.

8. Metrical Measures.-The basis of the metrical system of measures is the linear metre, a length equal to 39.368 American inches. To the decimal divisions of this length, names composed of the word metre and a prefix formed from Latin numerals have been given; and the decimal multiples of the same standard are similarly made up by engrafting Greek numerals. The following are the designations: one-tenth of a metre is called one decimetre; one-hundredth, one centimetre; one-thousandth, one millimetre; and ten metres are called one dekametre, one hundred one hectametre, one thousand one kilometre, etc.

The cubic decimetre, or litre, is the unit most generally used as the standard of volume, but the cubic centimetre is also very often employed. To compare these measures with one more familiar, it may be remembered that one litre is equal to 61.016 cubic inches or 2.113 pints.

FIG. 1.

9. Gravity. The attractive force by which bodies are drawn to the surface of the earth is called gravity. It acts between masses of matter of every kind, and at all distances. The mutual attraction of masses of matter has been thus illustrated: A pair of leaden balls, two inches in diameter, were attached to the ends of a rod, which was suspended in the middle by a fine wire (Fig. 1). Two other balls of lead, a foot in diameter, were placed upon a revolving platform, and, when the larger and smaller balls were brought near together, they were mutually attracted, as was shown by the motion of the rod. The force exerted did not exceed the twenty-millionth

Mutual Attraction of Leaden Balls.

of the weight of the lesser ball, but was sufficient to slightly twist the wire, and give rise to a small oscillatory movement. The force of gravity is proportional to the quantity of matter; that is, if the earth had twice its present mass, its attraction would be doubled, if but one-half its mass, its force would be only half as great. So with any body on the earth, the force with which it is attracted increases or diminishes in exact proportion to its quantity.

10. Weight.-If a body, instead of being allowed to fall, is supported, its tendency to descend is not destroyed. It is drawn downward with the same force, but, as it is resisted, and at rest, the force takes the shape of pressure. This downward pressure of bodies is called their weight. The weight of a body is the force it exerts in consequence of its gravity, and, as this force depends upon the quantity of matter, it is clear that, if the mass be doubled, the weight will be doubled; if the mass be halved, the weight will be halved. Weights are therefore nothing more than measures of the force of gravity in different objects, and we measure the force to determine the quantity of matter. 11. The Balance.-The instruments employed by chemists in weighing are balances. The chemical balance (Fig. 2), used for analysis, consists of an inflexible bar, delicately poised at a point exactly midway between its extremities, from which the scalepans are suspended. Its beam rests upon a fine edge of hardened steel, which is supported by a flat plate of polished agate. This beam

FIG. 2.

The Chemical Balance.

just as the rod in the toward the larger balls.

oscillates toward the earth preceding experiment cscillated

12. Standard Weights.-The operation of weighing consists in estimating the force with which any given body is attracted toward the earth by comparing it with other masses of matter already weighed and marked according to some fixed standard, as Troy, Avoirdupois, or French weight. These standard scales are quite arbitrary, there being no natural starting-point, or unit. The grain-weights were originally grains of wheat. The scales established in this country are capriciously arranged, while the French employ a decimal scale, which, being far more convenient, is almost always used in scientific investigations, and is gradually being adopted by different states and countries as the legal standard for the transaction of ordinary business.

13. Metrical Weights.-The French or metrical system of weights is based upon the metrical measures before mentioned. The standard unit of the scale is the weight of one cubic centimetre of pure distilled water at the temperature of maximum density (39°.2 Fahr.). The decimal fractions and multiples of the scale are distinguished by the addition of the same Latin and Greek prefixes already mentioned above, to the name of the unit. This is called the gramme, or gram. The gramme is equal to 15.432 grains, and the kilogramme to 22.046 pounds avoirdupois. Tables of equivalence of French and English weights are given in the Appendix.

§ 2. Specific Mass, Volume, and Weight.

14. Specific Volume.-Different bodies of equal weight do not occupy like amounts of space. Though a pound of cork exactly counterpoises a pound of lead, yet the former has a volume forty times greater than the latter. By comparing, therefore, the volumes of different substances with the volume of any one body of equal weight taken as unity, we may obtain their specific volumes.

15. Specific Weight or Gravity.-Inversely, also, dif