modified. It consists of several different forms of matter, and is decomposed; one part of its elements passes into the atmosphere as smoke, while another is left as a residue consisting of ash and charcoal. In short, the substance we know as wood has disappeared, and is replaced by others which are entirely different. The combustion of wood is then a chemical phenomenon.

2. Matter, mass, density.—We understand by the term matter whatever can affect one or more of our senses; that is to say, anything whose existence can be recognised by the sight, touch, taste, smell, or hearing.

The mass of a body is the quantity of matter contained in this body. Different substances may contain very different quantities of matter in the same volume. It will subsequently be shown, for instance, that for equal volumes lead contains nearly eleven times as much matter as water, and gold nineteen times as much. This is expressed by saying that the masses of lead and of gold are respectively eleven and nineteen times as dense as water. When one body has for the same volume twice or thrice the mass of another, it is said to be twice or thrice as dense; and the density of one substance in reference to another is the number which expresses how much matter the first body contains as compared with the second.

3. Simple and compound substances.--It has been ascertained that all the various forms of matter with which we are acquainted may be resolved into about sixty-five different kinds, which are called simple substances or elements, to express that each only contains one kind of matter. Many of these are very rare, and are found in very minute quantities; others are more widely diffused, and have important uses, but are not abundant; and the great mass of the universe is made up of about fourteen: the non-metallic bodies, or metalloids, oxygen, hydrogen, nitrogen, silicon, carbon, sulphur, phosphorus, and chlorine; and the metals aluminum, potassium, sodium, calcium, magnesium, and iron.

Very few of these elements occur in nature in the free state; by far the greater number of the substances we know are compound, that is, formed by the union of two, three, or four of these elements. Thus water consists of hydrogen and oxygen; marble, of carbon, oxygen, and calcium; muscular tissue, of carbon, hydrogen, oxygen, and nitrogen. The number of substances containing more than four elements is very small.

The force in virtue of which different substances unite to form compounds, and which opposes the resolution of compounds into their elements, is called the force of chemical attraction or affinity

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Different States of Matter.

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4. Internal constitution of bodies, atoms, molecules, molecular forces. The properties of bodies prove that they are not formed of continuous and compact matter as they seem to be, but that they are agglomerations of excessively small material particles, which are called atoms. The elementary atoms can unite with each other to form compounds, but cannot be destroyed by any known process.

The term molecule is given to the smallest cluster of atoms of any substance which is conceived capable of existing by itself; every pure substance consists of similar molecules.

The same properties which have led physicists to assume the existence of atoms and molecules, have also led to the assumption that these small particles do not touch, but are simply juxtaposed, retaining between them excessively small intervals, which we shall afterwards investigate under the name of pores (9).

But it may be asked, How is it that bodies do not spontaneously fall into powder? What gives them solidity and hardness? What is the invisible force that unites atoms and molecules?

This force is the reciprocal attraction which the molecules of bodies exert upon each other and which is continually drawing them together. The force which holds together particles of the same kind of matter is called molecular attraction; the force which holds together particles of different kinds of matter is called chemical attraction or affinity. When hydrogen and oxygen unite to form water they do so by reason of the exercise of the latter force, while the particles of water are held together by molecular attraction.

If molecular attraction were the only force acting upon the small particles of which bodies are composed, they would come into complete contact, which is never the case. They are also under the influence of a repulsive force, in virtue of which their particles continually tend to separate themselves; this is the force of heat. Experiment shows, in fact, that whenever a body is heated its volume increases because its molecules are driven apart; while on the contrary its volume diminishes when it is cooled, because the molecules then become closer. The particular form which matter assumes-whether solid, liquid, or gaseous-depends on the extent to which it is influenced by these antagonistic forces.

5. Different states of matter.-All different substances present characters in virtue of which they may be divided into three distinct classes, solids, liquids, and gases.

Solids, such as wood, stones, metals, &c., are substances which

are more or less hard, and retain the form which they possess naturally, or which has been given them by art. It is assumed that in solids molecular attraction preponderates over repulsion.

Liquids, such as water, oil, mercury, are bodies which have no hardness, and present but little resistance when a body is immersed in them; they have no shape of their own, but at once take that of the vessels in which they are contained; they are virtually incompressible. We assume that in them molecular attraction is balanced by the repulsive force of heat, and that while the molecules can freely glide over each other they keep an invariable distance apart if the temperature be not altered.

Gases, such as hydrogen, oxygen, carbonic acid are also called aeriform fluids from their analogy with our air, which is a mixture of oxygen and nitrogen. They are very light bodies; excepting a small number, which are coloured, they are invisible, and hence a vessel filled with air, hydrogen, or any colourless gas, appears quite empty. Like liquids, they have no shape of their own, but, unlike liquids, they are eminently compressible and expansible. In them the repulsive force of heat preponderates over molecular attraction (4); whence it follows that they are continually tending to occupy a larger space. This property will be described as the expansibility

of gases (110).

There are many bodies which can exist in these three different forms; thus water, exposed to great cold, becomes solid in the form of ice; at ordinary temperatures it is liquid, while at higher temperatures it becomes a gas. Sulphur, iodine, and several metals present the same phenomena.

CHAPTER II.

GENERAL PROPERTIES OF BODIES.

6. Extension. By general properties we understand those which are common to all bodies, whether solids, liquids, or gases ; such for instance are extension, impenetrability, divisibility, porosity, compressibility, elasticity, inertia, and gravity.

Specific properties are such as we observe only in certain bodies, or in certain states of those bodies; solidity, fluidity, tenacity, malleability, colour, hardness, etc. are properties of this class.

The first general property of bodies with which we are concerned

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Divisibility.

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is their extension or magnitude; that is, the extent of space they occupy. All bodies, even the smallest atoms, have a certain extension.

Extension considered in only one direction, that of length, gives a line; in two directions, length and breadth, a surface; and, in the three directions, length, breadth, and thickness, a volume.

With respect to the above general properties, it may be remarked that impenetrability and extension might be more aptly termed essential attributes of matter, since they suffice to define it; and that divisibility, porosity, compressibility, and elasticity do not apply to atoms, but only to bodies or aggregates of atoms.

7. Impenetrability. This is the property in virtue of which two portions of matter cannot simultaneously occupy the same portion of space. Strictly speaking, this property only applies to the atoms of bodies.

In many phenomena bodies appear to penetrate each other. Thus, if a pint of water and a pint of alcohol be mixed, the volume of the mixture is less than two pints. A similar contraction occurs, in the formation of certain alloys: thus brass, which is an alloy of copper and zinc, occupies a less volume than the united volumes of its constituents.

This penetration is, however, only apparent, and is due to an alteration in the position of the molecules; they come nearer each other, and the space occupied by the pores is diminished.

A nail driven into wood is not a case of penetration. The molecules of the latter are driven apart by the nail, but wherever it has penetrated there is no wood. When water has been poured upon a heap of sand it at once disappears; the water, however, does not penetrate the substance of the sand itself, but merely fills the interstices between the grains.

8. Divisibility. This is the property which all bodies have of being divided into distinct parts.

Numerous examples may be cited of the extreme divisibility of matter. The tenth part of a grain of musk will continue for years to fill a room with its odoriferous particles, and at the end of that time will scarcely be diminished in weight.

Blood is composed of red, flattened globules floating in a colourless liquid called serum. In man the diameter of one of these globules is less than the 3,500th part of an inch, and the drop of blood which might be suspended from the point of a needle would contain about a million of globules.

Again, the microscope has disclosed to us the existence of insects smaller even than these particles of blood; the struggle for existence reaches even to these little creatures, for they devour still smaller ones. If blood runs in the veins of these devoured ones, how infinitesimal must be the magnitude of its component globules?

Has then the divisibility of matter no limit? Although experiment fails to determine such limit, many facts in chemistry, such as the invariability in the relative weights of the elements which combine with each other, would lead us to believe that a limit does exist. It is on this account that bodies are conceived to be com

posed of extremely minute and indivisible parts called atoms (4).

:

distinguished physical or intermolecular pores, where the interstices are so small that the molecules remain within the sphere of each other's attracting or repelling forces; and sensible pores, or actual cavities, across which these molecular forces cannot act.

The contractions and dilatations resulting from variations of temperature are due to the existence of physical pores; whilst in the organic world the sensible pores are the seat of the phenomena of exhalation and absorption.

In wood, sponge, pumice stone, and in animal and vegetable tissues, the sensible pores are apparent; physical pores never are. Yet, since the volume of every body may be diminished, we conclude that all possess physical pores.

Fig. 1.

The existence of sensible pores may be shown by the following