Page images
PDF
EPUB

NATURAL PHILOSOPHY.-ELEMENTARY PRINCIPLES.

CHAPTER I.

PROPERTIES OF MATTER.-The Three Forms of Matter-Vapours-The distinctive, essential, and accessory properties-Extension-Impenetrability-Unchangeability-Illustrations of Extension-Methods of measuring small spaces-The Spherometer-Illustration of Impenetrability-The Diving Bell-The accessory properties of Matter-CompressibilityExpansibility-Elasticity-Limit of Elasticity—Illustrations of Divisibility-Porosity and interstitial spaces-Weight.

MATERIAL substances present themselves to us under three different conditions. Some have their parts so strongly attached to each other that they resist the intrusion of external bodies, and can retain any shape that may be given them. These constitute the group of SOLIDS. A second class yields readily to pressure or movement, their particles easily sliding over one another; and from this extreme mobility they are unable of themselves to assume determinate forms, but always copy the shape of the receptacles or vessels in which they are placed they are LIQUIDS. A third, yielding even more easily than the foregoing, thin and aërial in their character, and marked by the facility with which they may be compressed into smaller or dilated into larger dimensions, give us a group designated as GASES. Metals may be taken as examples of the first; water as the type of the second; and atmospheric air of the third of these states or conditions, which are called "the THREE FORMS of bodies."

In some instances the same substance can exhibit all three of these forms. Thus, when liquid water is cooled to a certain degree, it takes on the solid condition, as ice or snow; and when its temperature is sufficiently raised, it assumes the gaseous state, and is then known as steam. Writers on Natural Philosophy have found it convenient, for many reasons, to introduce the term Vapours, meaning by that a gas placed under such circumstances that it is ready to assume the liquid state. As the steam of water conforms to this condition, it is therefore regarded as a vapour.

Under whichever of these forms material substances are presented, they exhibit certain properties—these are, first, Distinctive; second, Essential; third, Accessory.

There is a certain bright white metal passing under the name of Potassium, the distinctive character of which is, that, when thrown on the surface of water, it gives rise to a violent reaction, a beautiful violet-coloured flame being evolved. A piece of lead, which to external appearance is not unlike the potassium, when brought in contact with water exhibits no such phenomenon, but, as every one knows, remains quietly, neither disturbing the water nor being acted upon by it.

Fig. 1.

Such distinctive qualities are the objects of a chemist's studies. It belongs to his science to show how some gases are coloured and others colourless; some supporters of combustion, while others extinguish burning bodies; how some liquids can be decomposed by voltaic batteries and some by exposure to a red heat. The general doctrines of affinity, the modes in which

bodies combine, and the characters of the products to which they give rise -all these belong to Chemistry.

But beyond these distinctive qualities of bodies, there are, as has been observed, certain other properties which are uniformly met with in all bodies whatever, and hence are spoken of as ESSENTIAL. They are,

Extension.
Impenetrability.

Unchangeability.

By EXTENSION we mean that all substances, whatever their volume or figure may be, occupy a determinate portion of space. We measure them by three dimensions-length, breadth, and thickness.

[ocr errors]

d

IMPENETRABILITY points out the fact that two bodies cannot occupy the same space at the same time. If a nail is driven into wood, it enters only by separating the woody particles from each other; if it be dropped into water, it does not penetrate, but displaces the watery particles: and even in the case of aërial bodies, through which masses can move with apparently little resistance, the same observation holds good. Thus, if we take a wide-mouthed bottle, a, Fig. 2, and insert through its cork a funnel, b, with a narrow neck, and also a bent tube, c, which dips into a glass of water, d, on pouring any liquid into the funnel, so that it may fall, drop by drop, into the bottle, we shall find, as this takes place, that air passes out, bubble after bubble, through the water in d. The air is, therefore, not penetrated by the water, but displaced.

Fig. 2.

The same fact may also be proved by taking a cuppingglass, a. Fig. 3, and immersing it, mouth downwards, in a glass of water, b. If the aperture, c, of the cuppingglass be left open, the air will rush out through it, and flow in below: but if it be closed by the finger, as the air can now no longer escape, the water is unable to enter and occupy its place.

Similar experiments establish the impenetrability of liquids by solids. If in a glass of water, Fig. 4, a leaden bullet is immersed,

[graphic][merged small]

it will be seen that as the bullet is introduced the water rises to a higher level, showing, therefore, that a liquid can no more be penetrated by a solid than, as was seen in the former experiment, can a gas by a liquid. Two bodies cannot occupy the same space at the same time.

The third essential property of matter is its UNCHANGEFig. 4. ABILITY. This property may be. looked upon as the foundation of Chemistry; and though there are many phenomena which we constantly witness which seem to contradict it, they form, when properly considered, strikingfillustrations of the great truth that material substances can neither be created nor destroyed; and that the distinctive qualities which appertain to them remain for ever unchanged. The disappearance of oil in the combustion of lamps, the burning away of coal, the evaporation of water, when minutely examined, far from proving the perishability of matter, afford the most striking evidence of its duration. Nor is a solitary fact known in

the whole range of Chemistry, Natural Philosophy, or Physiology, which lends the remotest countenance to the opinion that, either by slow lapse of time or by any artificial processes whatever, can matter be created, changed, or destroyed. Even the bodies of men and animals, the structures of plants, and all other objects in the world of organization, which seem characterized by the facility with which they undergo, unceasing and, eventually, total change, are no exceptions to the truth of this observation. The bodies which we possess to-day are made up of particles which have formed the · bodies of other animals in former times, and which will again discharge the same duty for races that will hereafter come into existence.

As illustrations connected with the extension and impenetrability of matter, I may give the following instances:

We are frequently required to measure the dimensions of bodies; that is, to determine their length, breadth, or thickness. It is a much more difficult thing to do this accurately than is commonly supposed. It requires an artist of the highest skill to make a measure which is a foot or a yard in length, or which shall contain precisely a pint or a gallon. With a view of facilitating the measurement of bodies, à great many contrivances have been invented, such as verniers, spherometers, and screw machines of different kinds.

m

a

The spherometer, which is a beautiful contrivance for measuring the thickness of bodies, is constructed as follows: It has three horizontal steel branches, a, b, c, Fig. 5, which form with each other angles of 120 degrees. From the extremities of these branches there proceed three delicate steel feet, d, e, f, and through the centre, where the branches unite, a screw, g, the thread of which is cut with great precision, and which terminates in a pointed foot, i, passes. The head of this screw carries a divided circle, m. Now, suppose the instrument is placed on a piece of flat glass, it will be supported on its three feet, which are all in the same plane; but if in turning the screw we depress its point, i, beneath the plane of its feet, it can no longer stand with stability on the glass, but totters when it is touched, and emits a rattling sound. By altering the screw, therefore, we can give it such a position that both by the finger and the ear we discover that its point is level with the points d, e, f. Now let the object, the thickness of which is to be measured, be placed on the glass, and the screw turned until the instrument stands without tottering; it is obvious that its point must have been lifted through a distance precisely equal to the thickness of the object to be measured, and the movement of the head of the screw, read off upon the scale n, against which it works, indicates what that thickness is.

Fig. 5.

This instrument, therefore, serves to show that in the measurement of small spaces, the senses of touch and hearing may often be resorted to with more effect than the eye. The spherometer is here introduced in connection with these general considerations respecting the extension of matter, as affording the student an illustration of the delicate methods we possess of determining the minutest dimensions of bodies.

As an illustration of the impenetrability of matter, the machine which passes under the name of the diving-bell may be mentioned. It consists of a vessel, a a, Fig. 6, of any suitable shape, and heavy enough to sink in water when plunged with its mouth downward. Owing to the impenetrability of the air, the water is excluded from the interior, or only finds access to such an extent as corresponds to the pressure of the depth to which it is sunk. Light is admitted to the bell through thick pieces of glass in its top, and a constant stream of fresh air thrown into it from a tube, b, and forcing-pump above, the atmosphere in the inside being suffered to escape through a stop-cock as it becomes vitiated by the respiration of the workmen. Diving-bells are extensively resorted to in submarine architecture, and for the recovery of treasure lost in the sea.

A

Fig. 6.

Having disposed of the essential, we pass next to a consideration of the accessory properties of matter. They are,

Compressibility.
Divisibility.

Expansibility.
Porosity.

Elasticity.
Weight.

That substances of all the three forms are compressible is capable of easy proof. In the process of coining, pieces of metal are exposed to powerful pressure between the steel dies, so that they become much denser than before. By enclosing water or any other liquid in a strong vessel, and causing a pison, driven by a screw, to act upon it, it may be reduced to a less space; and gaseous substances, such as atmospheric air, when enclosed in an indiarubber bag, or even a bladder, may be compressed by the hands.

a

C

Fig. 7.

[ocr errors]

Under the influence of heat all substances expand. This may be proved for such solids as metals, by the apparatus represented in Fig. 7. It consists of a stout board, a, b, on which are fastened two brass uprights, c, d, with notches cut in them, so as to receive the ends of a metallic bar, e. This bar is slightly shorter than the whole distance between the notches, so that when it is set in its place, it can be moved backward and forward, and emits a rattling sound. But if boiling water be poured upon it, it expands and occupies the whole distance, and can no longer be moved. [A very simple manner of illustrating the expansion of metals by heat is showing the following diagram. A plate of brass or other metal is made of the same shape as the upper one in the above figure, and a rod of metal-the same kind as the plate-is fixed to a metal stem inserted in a wooden handle, as d c. The rod is made to fit the hole b, and also the space a, between the two ends. When the metallic rod is plunged into hot water, or heated over the flame of a spirit-lamp, the expansion is so great that it becomes impossible to pass the end of the rod through the hole bor the space a.

Fig. 8.

The expansion of liquids is well shown in the case of common thermometers, which contain either quicksilver or spirits of wine, those sub

stances occupying a greater volume as their temperature rises. The airthermometer proves the same thing for gases.

By elasticity we mean that quality by which bodies, when their form has been changed, endeavour to recover their original shape. In this respect there are great differences. Steel, ivory, india-rubber, are highly elastic; lead, putty, and clay, less so. Perfectly elastic bodies resist the action of disturbing causes without any ulterior change; thus, a quantity of atmospheric air, compressed into a copper globe, recovers its original volume as soon as the pressure is removed, though it may have been shut up for years. By the limit of elasticity we mean the smallest force which is required to produce a permanent disturbance in the structure of an imperfectly elastic body. No solid is perfectly elastic. An iron wire, drawn a little aside, recovers its original straightness; but if more violently bent, it takes a permanent set, because its limit of elasticity is overpassed. The elasticity of a given substance can often be altered by mechanical processes, such as by hammering, or by heating and cooling, as in the process of tempering.

The divisibility of matter may be proved in many ways. By various mechanical processes, metals may often be reduced to an extreme degree of tenuity; thus it is said, that gold-leaf may be beaten out until it is only 200000 of an inch thick. By chemical experiments, a grain of copper or of iron may be divided into many millions of parts. For certain purposes artists have ruled parallel lines upon glass, with a diamond point, so close to each other, that ten thousand are contained in a single inch. The odours which are exhaled by strong-smelling perfumes, as musk, will for years together infect the air of a large room, and yet the loss of weight by the musk is imperceptible. Again, there are animals whose bodies are so minute, that they can only be seen by the aid of the microscope. The siliceous cells of such infusorials occur in many parts of the earth as fossils. Ehrenberg has shown that tripoli, a mineral used in the arts, is made up of these—a single cubic inch of it containing about forty-one thousand millions—that is, about fifty times as many individuals as there are of human beings on the face of the globe.

As substances of all kinds may be reduced to smaller dimensions, either by pressure or the influence of cold, and as it is impossible for two particles to occupy the same place at the same time, or even for one of them partially to encroach on the position occupied by the other, it necessarily follows that there must be pores or interstices even in the densest bodies.

Fig. 9.

[The annexed diagram will illustrate the interstitial spaces between the atoms of bodies, and this may be further demonstrated by the following simple experiment: Place marbles in a jar until it will not hold any more, and you will find that there are spaces between the marbles, because you may afterwards add shot and sand, and finally water.]

Thus quicksilver will readily soak into the pores of gold, and gases ooze through india-rubber. Writers on Natural Philosophy usually restrict the term "pore" to spaces which are visible to the eye, and designate those minute distances which separate the ultimate particles of bodies by the term "interstices."

All bodies have weight or gravity. It is this which causes them to fall, when unsupported, to the ground; or when supported, to exert pressure

« PreviousContinue »