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a is the cylinder, b the connecting rod, c the crank, d eccentric, by which the slide valve is moved; e e e is the steam pipe by which the steam is conducted from the steam dome of the boiler to the cylinder. Near the furnace end of this pipe is a valve or regulator moved by a handle at the front of the boiler, and of which the purpose is to regulate the admission of the steam to the cylinder; ff are safety valves kept closed by springs; g is the eduction pipe, or, as it is commonly termed in locomotives, the blast pipe, by which the steam, escaping from the cylinder after the stroke has been performed, is projected up

the chimney H. The water in the boiler of course covers the tubes, and also the top of the furnace or fire-box. The position of the water surface is shown by a dotted line. It will be understood that there are two engines in each locomotive, though, from the figure being given in section, only one engine can be shown. The cylinders of this engine are each 12 inches diameter; the length of the stroke of the piston is 18 inches, and the diameter of the driving wheel is 5 feet.

Figs. 61. and 62. are vertical and horizontal sections of the locomotive engine of Messrs. Hawthorn of Newcastle as constructed by Fig. 61.

MATTER AND ITS PROPERTIES.

MATTER is anything extended, and capable of resisting or transmitting force. A body is a quantity of matter, and every body has length, breadth, and thickness, and resists in a greater or less degree any pressure applied to it. It follows from this definition that a body may not be a substance, and that there may be no such thing in nature as a substance at all. Our ideas of a substance we acquire through the medium of our senses, and any cause which produced the proper sensation would also produce the corresponding idea, whether the substance existed or not. The recent investigations of Professor Faraday and other experimentalists have rendered it probable that matter is only an aggregation of centres of force. But all the laws relating to matter hold equally true, whether this be its real character, or whether it is an actual substance, as commonly believed.

The inherent properties of matter are volume, inertia, mobility, and divisibility. A fifth property is attraction; but it appears to be rather incidental than essential. Of attraction there are four varieties, namely, gravitation, cohesion, magnetism, and electrical attraction; and there appears little reason to doubt that these are but the different phases of one elementary principle, which is motion.

Every body must have size or volume, and the limit or termination of a body is a surface or superficies. Thus a dice or cubic inch of ivory has for its superficies six planes, each measuring one square inch. These surfaces meet in edges or lines each one inch long, and the edges meet in corners or points which are destitute of dimension. No finite or imaginable division of a line can ever produce a point, and a line is therefore divisible into an indefinite number of other lines, each of which will have a length inversely proportioned to the number of divisions. In the same way no subdivision of a surface can ever produce a line, and no subdivision of a solid can ever produce a surface. It hence becomes obvious that any quantity of matter, however small, may be made to fill any space, however large, without any pore or interstice occurring, whose diameter shall exceed a given finite line. A cubic inch of ivory or of iron might be enlarged into a cube of the diameter of the earth, and yet the matter be equally diffused throughout the space, and be without hole or vacuity larger than the smallest grain of sand; and this enlarged cube might again be compressed into its original volume without suffering any diminution in the quantity of matter of which it is composed. A cubic inch of water will, by the application of heat, be converted into a cubic foot of steam of the same elasticity as the atmosphere, and by the abstraction of the heat the steam will be again turned into a cubic inch of water. The same effect is no doubt producible by mechanical compression, without any abstraction of heat at all; and as all bodies are compressible by the application of a suitable force, it follows that either the particles of matter are themselves compressible, or that they are not in contact in the natural state of bodies. Now if the particles of matter are themselves compressible, it is clear that the larger the compressing force is, the greater must be the amount of the compression; so that by increasing the compressing force sufficiently, the matter would be forced into a smaller and smaller space, until it disappeared altogether. If, on the contrary, matter be composed of incompressible particles, the further compression of any body will be impossible when the particles of matter composing the body have been brought into contact with one another; and a body in such a state will be incapable of contracting

with a diminution of temperature, as it is already in the smallest com. pass in which it can exist. Thus, if a thousand cubic inches of gold be supposed to contain one cubic inch of matter, the further condensation of gold by pressure will be impossible, after it has been forced into a space equal to one-thousandth of its original bulk; and matter in such a state of density will be incapable of contraction by any amount of cold. As water is about nineteen times lighter than gold, the matter in a thousand cubic inches of water will occupy one-nineteenth of a cubic inch when the particles of matter are in contact ; and the sum of the vacuous spaces in the gold will be nine hundred and ninety-nine cubic inches, and in the water nine hundred and ninety-nine cubic inches and eighteen nineteenths of a cubic inch; or, in other words, the sum of the vacant spaces in the water, and in the gold, will be very nearly the same, if gold contain the quantity of matter that we have assumed for the purpose of illustration.

Whether matter be a real substance, or an aggregation merely of centres of force, it appears to be at least certain that the ultimate particles or atoms of matter are indivisible—at least, without changing their nature- for, although a space, however small, is obviously divisible into any number of smaller spaces, and although an atom if occupying space, or if constituting a force, may be supposed to be similarly divisible, yet we have no experience of such a division actually occurring in nature. The laws of atomic combination render it certain that it is only between the atoms of bodies, and not between parts of atoms, that action and reaction take place; and although it is conceivable that matter, if consisting of centres of force, may be resolved into some equivalent force manifesting different qualities, and that parts of atoms may consequently assume such a shape, yet in such a case the parts of atoms are no longer matter, and the supposition does not invalidate the doctrine that the atoms of matter are indivisible. The solid, liquid, and vaporous states of bodies are conditions accidental to their temperature; for every solid may be melted and raised into vapour by an adequate heat, and every vapour or gas may be liquefied or solidified by an adequate cold. If a small quantity of gas or vapour be admitted into a large vacuous space, it will expand until the weight of the particles just balances their repulsive force; and at the point where this equilibration takes place, the particles will arrange themselves in a level plane, like the surface of a sheet of water. If an electric spark be transmitted through a Torricellian vacuum made over mercury, in a long glass tube, a pale light will be visible in those parts of the vacuum near the mercury, on account of the existence of mercurial vapour there. But if the tube be very long, and be also maintained at a low temperature, there will be a plane in the vacuous space, beyond which the mercurial vapour cannot ascend; and this plane may be ascertained by transmitting sparks through the tube, at points higher and higher up, until such a point is reached that no light is produced within the tube when a spark is transmitted. At this point the weight of the particles of the mercurial vapour will just balance their repulsive force.

Impenetrability is commonly reckoned one of the properties of matter, but from the tenor of the foregoing remarks it will be obvious that, although in a modified sense impenetrability must continue to be regarded as one of the characteristics of matter, whatever theory relative to the constitution of matter may be adopted, yet in the commonly received sense it may be totally untrue. By impenetrability is meant, not hardness or inseparability, but the impossibility of two bodies

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Matter and its Properties. being in the same place at the same time; and if matter be an actual substance, the attribute of impenetrability is undoubtedly just. For although all bodies may contain other bodies in their pores or interstices, in the manner in which water exists in the pores of wood, air in the pores of water, and quicksilver in the pores of gold, yet it is obviously not the matter itself in the wood, water, or gold which is thus penetrated, but merely vacuous spaces in it, which become filled with different matter, or matter in a different state. If matter, however, be not an actual substance, but is merely a collection of centres of force, it may be compressed into a smaller and smaller bulk, until the several centres of force become coincident; but the effect of such a compression would be to change it from the principle which we recognise as matter into some other thing. Although, therefore, matter must be impenetrable in a certain sense, inasmuch as if penetration were carried far enough the matter would cease to exist, we are not warranted in asserting that any amount of penetration is impossible, supposing the result to be disregarded. It is by no means certain that bodies resist pressure because their particles occupy space; and in the case of two electrified balls which repel one another, though some distance asunder, we see a resistance exerted, which, if capable of acting on the sense of touch, would no doubt produce the sensation of contact. If the power of repulsion be supposed to be sufficiently great, it will follow that ordinary natural agents will be unable to overcome it, and the phenomena of impenetrability will thus take place, though the matter itself may not be impenetrable or even extended. Since then we know that spheres of repulsion exist, and that the centres of those spheres are by virtue merely of such repulsion effectually kept asunder, and since it is superfluous to adduce more causes of natural things than are necessary for the explanation of the observed phenomena, it does not appear necessary that we should suppose an extended atom to exist in the centre of the sphere of repulsion, inasmuch as all the observed phenomena are explicable without such a supposition.

One of the most important properties of matter is that of inertia, by which is meant an incapacity of spontaneous change of place. All matter must be either at rest or in motion, and whatever be its condition in this respect, such condition can only be changed by the application of some external and counteracting force. There cannot indeed be any such thing as absolute rest in creation, for every particle of matter in the universe is known to be in perpetual motion, but a body may be at rest relatively with the earth, or with other bodies; and it is this relative state that is signified when rest is spoken of in physical dissertation. To put a mass of matter in motion requires the exertion of force, and to stop the motion requires the exertion of an equal and opposite force. A pendulum impelled by gravity, or any other force, through the descending arc of a circle, will, by virtue of its inertia or momentum, rise through the ascending arc to the level from which it originally fell; or rather it will ascend nearly, and would ascend quite, to the original level but for friction and the resistance of the air which occasion some retardation. The air, however, cannot resist a moving body without being put in motion itself, so that although there is less velocity or intensity of motion in the moving body in consequence of the atmospheric resistance, there is a greater quantity of matter moved than would be the case if there were no atmosphere present. If a given force moves a certain quantity of matter with a given velocity, it will require twice the force to move twice the quantity of matter with the same velocity; but twice the force will not move the original quantity of matter with twice the velocity, as is sometimes asserted, and of which we shall investigate the reason when we come to speak about falling bodies. A ball rolled along the ground comes in a short time into a state of rest, in consequence of the friction of the rolling surfaces and the resistance of the atmosphere; but if the atmosphere were removed, and the ball and the plane on which it rolls were quite smooth, the ball would roll on for ever, and would circulate round the earth continually, supposing the plane to be so extended as to encircle the earth like an equator. By friction, heat and electricity are excited, and in this way a part of the force communicated to any moving body may be lost for useful any purpose. It does not, however, follow that any part of the force is absolutely annihilated, any more than the water is annihilated which escapes from a leaky vessel, and all that has been asserted of the indestruct

Matter not a distinct Entity.

ible nature of matter appears to hold true of the indestructible nature of force, which there is every reason to believe can neither be created nor destroyed. Bodies consequently which are brought to a state of rest by friction and atmospheric resistance, are not influenced by any more natural aptitude for rest than for motion; but they merely lose motion in the proportion in which they lose force by its dissipation in friction, and in putting into motion other bodies than those to which the force was primarily applied.

The quantity of matter existing in any body is always reckoned as proportional to the inertia or weight of the body. In bodies of the same kind the quantities of matter are determinable by their respective volumes; but in bodies of different kinds the quantities of matter are only determinable by their volumes and densities conjointly, or by the inertia or weight. Thus, we know that a cubic inch of gold has just one-tenth of the matter in it that ten cubic inches of gold have; but a cubic inch of gold is said to have one-fifth of the matter in it that ten cubic inches of copper have, since ten cubic inches of copper have only the same weight or inertia as five cubic inches of gold. This doctrine, however, of the quantity of matter in bodies being as their weight, though commonly accepted, is by no means demonstrable, and is perhaps untrue; for the weights of atoms of matter may vary from differences in their attractive force, as well as from differences in their size and density; or the total weight of a dense body may be made up of a small number of particles, each with a large attraction, as well as of a large number of particles, each with a small attraction. The physical result, however, is the same, whichever supposition is adopted; and it will be more convenient, and more conformable to established usage, to regard the quantity of matter in a body as proportional to its weight.

The term density expresses the relative quantities of matter in different bodies of the same volume or size. For instance, a cubic foot of cast-iron weighs about 450 pounds, and a cubic foot of water weighs 62 pounds. The density of cast-iron, therefore, is 7.2 times greater than the density of water. Rarity is the reverse of density. Thus water is 7.2 times rarer than cast-iron. The term specific gravity denotes the density of any substance relatively with the density of water; or, in other words, it denotes the weight of any volume of such substance compared with the weight of an equal volume of water. Thus the specific gravity of cast-iron will be about 7.2, the specific gravity of water being 1.

When it is found that the several parts of a body are alike in every respect, we may safely conclude that they are alike in weight. Such bodies are said to be homogeneous,-or of uniform density,― their weight being proportionate to their volume. Hence, by ascertaining the weight of some determinate volume of a homogeneous substance as a standard, we may take any multiple or fraction of that volume as being a corresponding multiple or fraction of the total weight. Thus, if a cubic foot of water weigh 62 pounds, we know that a cubic inch will weighth part of that amount; or 1728 cubic feet will weigh 108,000 pounds. We are thus enabled to ascertain the weight of any given mass of water, when we know its volume, without weighing it on every separate occasion. In like manner the weights of masses of any other substance of which the density or specific gravity is known, may be readily determined.

MATTER NOT A DISTINCT ENTITY.

The metaphysical speculations which have been advanced at various times, showing the non-existence of matter, have heretofore been regarded by ordinary men as little else than feats of scholastic legerdemain, not intended to conciliate acquiescence. But the researches of Faraday and other experimentalists in various departments of material science, pointing as they do to the same conclusion, have unsettled that conventional dogmatism into which opinion had subsided, and have recalled the public attention to those fundamental considerations upon which, and not upon either authority or general acceptation, our judgments ought to rest. In the "Philosophical Magazine" for February 1844, there is a paper by Professor Faraday entitled "A Speculation touching Electric Conduction and the Nature of Matter," in which many weighty arguments are adduced to show that matter of the kind usually believed in, and which consists of an