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Good friends! sweet friends! let me not stir you
They that have done this déed' are honourable.
I am no òrator, as Brútus is,
But, as you know me áll, a pláin' blùut mán,
Cohesion, (con, haereo, L.) Lit. a stick-
Divisible, indivisible, divisibility, (di-
MATTER AND FORCE.
Atom, la, temno, G.) a particle so small | Friction, (frico, L.) Lit. rubbing; the
that it cannot be divided.
Attraction, (ad, traho, L.) Lit. a draw
resistance to a body moving on another, from the roughness of their surfaces.
PROPERTIES OF MATTER.
OUR senses make us acquainted with an immense varietyof substances, existing around us in different forms, and acting in many ways upon one another. There are the solid rocks, the liquid ocean, and the gaseous atmosphere; the hard diamond, and the soft clay; the odours of flowers, the dew-drops on the grass, the damp, raw mist which envelops our Scottish hills, the eternal snow upon the Alps, and the burning lava of Etna and Vesuvius. All these, and the hundreds of other substances which our senses reveal to us, are included under the general name of matter. The stones are matter, the stream is matter, the cloud is matter, the atmosphere itself is matter. Every object, in short, which we can perceive by any of our senses, is composed of matter, or, in other words, is a material object. There is within us something that is not material, a living, thinking, yet unseen being, to which we give the name of soul or spirit. And we are taught in Scripture that there are other spirits, whom we perceive not, though they may be near us; nay more, that God himself, the Creator of all, is not a material, but a spiritual Being. The word matter, then, is opposed to spirit; the word material, to spiritual.
There are certain properties which distinguish matter in all its various forms; these are called its general properties. The first and most obvious is, that matter occupies space, or, to use the scientific term, that it possesses the property of extension. This seems scarcely to need explanation. It implies length, breadth, and depth or height; for depth and height are manifestly the same, only that depth is measured from the top of an object downwards, and height from the bottom upwards. Extension implies also shape or figure, which is determined by the three dimensions already referred to.
Since matter always occupies a certain space, whether more or less, it is natural to infer, that two substances cannot occupy the same space at the same time. Experience
proves that this inference is correct. Let the hand, for example, be plunged into a glass of water, the water will be displaced, and will rise in the glass higher than it was before. If the glass be full before the hand is introduced, a portion of the water will run over. The water resigns its place to the solid body, and seeks a new position, but the two do not at the same time occupy the same space. Even so is it when a nail is driven into a piece of wood. Little pores or interstices exist between the woody particles, and the nail makes room for itself by squeezing these particles more closely together. The wood is accordingly displaced from the space which the nail now occupies. Air itself is able to prevent any other substance from occupying the same place with it. If a cup or glass be plunged into water with its mouth downwards, so as to be completely submerged, the air it contains, having no means of escape, will keep the water from filling it. This may be easily proved by sticking a little bit of paper on the bottom of the glass inside, which will be found quite dry when the glass has been withdrawn from the water. It is upon this principle that diving-bells are constructed. It is equally true, then, of all kinds of matter, solid, liquid, or gaseous, that the space which is occupied by one substance cannot at the same time be occupied by another. This property is generally, though not very correctly, called impenetrability.
It is to be particularly observed, that, when a nail is driven into wood, the woody particles surrounding it are crushed into less space than they filled before. So also, in the experiment just described, of plunging an inverted glass into water, it is found that the water, though it does not fill the glass, rises in it to a certain extent. The air in the glass is compressed into a smaller space than it originally occupied, and its bulk becomes less and less, as the glass is pressed deeper and deeper into the water. Here, then, we have examples of another property of matter, compressibility. Air possesses this property in a high degree, wood in a lower degree, and some other substances
in a lower degree still; but there is good reason to believe that everything material is more or less compressible.
The dimensions of a body are affected by heat as well as by pressure. Bars of metal alternately heated and cooled, will alternately expand and contract. The blacksmith takes advantage of this in fitting the iron rim to a wheel. He puts on the rim when expanded by heat, so that, gradually contracting as it cools, it holds the wood-work with a firm grasp. Liquids also expand under the influence of heat, and contract when exposed to cold, and the same phenomena are exhibited by gases in a much higher degree. Putting out of view, therefore, a few exceptional cases, it may be said generally, that matter, in all its forms, possesses the properties of dilatability and contractibility. Hence the substances around us are constantly swelling and contracting under the vicissitudes of heat and cold. They grow smaller in winter, and dilate in summer. They swell on a warm day, and contract on a cold one. Even our own bodies are not altogether exempt from these changes, but the effects of temperature on them involve a good deal more than mere contraction and expansion.
PROPERTIES OF MATTER-CONTINUED.
SEVERAL of the properties of matter mentioned in last lesson, seem to depend on one which has also been already referred to, viz., its porosity. It scarcely admits of doubt that all bodies are composed of incredibly minute particles or molecules, between which there are little spaces called pores, capable of enlargement and diminution. In other words, every kind of matter is more or less porous, and by increasing or diminishing its porosity it may be dilated or compressed. Closely connected with porosity is another property called density. It is important that this term should be well understood. One substance is more dense than another, if a given bulk of the former contains more
matter, or (which is practically the same thing) weighs more than the same bulk of the latter. Hence the density of a substance is increased by compression or contraction, and diminished by dilatation. In ordinary cases, density is measured by weight; what weight itself is, will appear in a subsequent lesson.
Another remarkable property of matter is its divisibility. A lump of marble may be ground into a fine powder, but each grain is still a mere block of marble, capable of further division, if instruments could be found fine enough to divide it. It is a curious question, whether there is, in matter itself, any necessary or absolute limit to this process. The general opinion is, that the molecules already spoken of are ultimate and indivisible. We are able, by artificial means, to reduce some kinds of matter, especially the metals, to particles of surprising minuteness; but we seem to be still very far from the point, if there really be such a point, beyond which no further subdivision is possible. It is needless to state in figures the immense number of parts into which a small quantity of gold, for example, may be divided. "A million" is easily said, but the idea of a million of objects, or of the millionth part of an ounce of gold is not so easily realized. When, therefore, we have learnt that it is possible to divide an ounce of gold, not into a million of parts only, but into hundreds of millions, we may well ponder with amazement the stupendous fact, but we shall utterly fail to form any adequate conception of its meaning. The mind is overwhelmed by the contemplation of so great a number. Yet each of the parts thus obtained is visible to the naked eye, and retains all the essential qualities of the largest masses of the same metal.
Foremost in importance among the properties of matter, at least in the study of mechanics, is that known as inertia. Every material body must be either at rest or in motion. Now inertia consists in this, that, if a body be at rest, it cannot put itself in motion, and if it be in motion, it cannot change that motion nor reduce itself to rest. The first of these two statements, namely, that a body at rest cannot