Page images
PDF
EPUB

upon the supporting body. Nor is this property limited to terrestrial objects; for in the same way that an apple tends to fall to the earth, so too does the moon; and all the planets gravitate toward each other and toward the sun. It was the consideration of this principle that led M. Leverrier to the discovery of a new planet beyond Uranus-this latter star being evidently disturbed in its movements by the influences of a more distant body hitherto unknown,

CHAPTER

PHYSICAL FORCES.

II.

Attractive and Repulsive Forces-Molecular Attraction-Gravitation

Cohesion-Constitution of Matter.

ALL changes taking place in the system of nature are due to the operation of forces. The attractive force of the earth causes bodies to fall, and a similar agency gives rise to the shrinking of substances--their parts coming closer together when they are exposed to the action of cold. In like manner, when an ivory ball is suffered to drop on a marble slab, its particles, which have been driven closer to one another by the force of the blow, instantly recover their original positions by repelling one another; that is to say, through the agency of a repulsive force. Of the nature of forces we know nothing. Their existence only is inferred from the effects they produce; and according to the nature of those effects, we divide them into ATTRACTIVE and REPULSIVE FORCES the former tending to bring bodies closer together, the latter to remove them further apart.

It has been found convenient to divide attractive forces into three groups, according as the range of their action or the circumstances of their development differ. When the attractive influence extends only to a limited space, it is spoken of as molecular attraction; but the attraction of gravitation is felt throughout the regions of space. By cohesion is meant an attractive influence called into existence when bodies are brought to touch one another. It is to be understood that these are only conventional distinctions; and it is not improbable that all the phenomena of attraction are due to the agency of

one common cause.

Chemists have shown that, in all probability, material substances are constituted upon one common type. They are made up of minute, indivisible particles, called atoms, which are arranged at variable distances from each other. These distances are determined by the relative prevalence of attractive and repulsive forces, resident in or among the particles themselves; and so too is the form of the resulting mass. If the cohesive predominates over the repulsive force, a solid body is the result; if the two are equal, it is a liquid; and if the repulsive prevails, it is a gas.

There are many reasons which lead us to suppose that the repulsive force, which thus tends to keep the particles of matter asunder, is the agent otherwise known as heat. Whenever the temperature of a body rises, it enlarges in volume, because its constituent particles move from each other, and on the temperature falling, the reverse effect ensues. If, as many very eminent

philosophers believe, heat and light are in reality the same agent, it follows, by a necessary consequence, as will be gathered from what we shall hereafter have to say on optics, that the atoms of bodies vibrate unceasingly, and that instead of there being that perfect acquiescence among them which a superficial examination suggests, all material substances are the seat of oscillatory movements, many millions of which are executed in the space of a single second of time: the number increasing as the temperature rises, and diminishing as it falls.

CHAPTER III.

OBSERVATIONS ON NATURAL PHILOSOPHY.

PNEUMATICS.-General Relations of the Air-Its connection with Motion and Organization Limited Extent-Constitution-Compressibility-Causes which Limit the Atmosphere-Its Variable Densities-Proportionality of its Elastic Force and Pressure.

A VERY superficial knowledge of those parts of the world to which man has access, readily leads to their classificaton under three separate heads-the Air, the Sea, and the solid Earth. This was recognised in the infancy of science; for the four elements of antiquity were the divisions which we have mentioned, and Fire.

NATURAL PHILOSOPHY or PHYSICAL SCIENCE, which, in its extended acceptation, means the study of all the phenomena of the material world, may commence its investigations with any objects or any facts whatever. By pursuing these, in their consequences and connections, all the discoveries which the human mind has made in this department of knowledge might successively be brought forward. But when we are left to select at pleasure our point of commencement, it is best to follow the most natural and obvious course. All the advances made in our times by the most eminent philosophers, and our powers of appreciating and understanding them, depend on clearness of perception of the great fundamental facts of science-a perspicuity which can never arise from mere abstract reasonings or from the unaided operations of the human intellect, but which is the natural consequence of а familiarity with absolute facts. These serve us as our points of departure, and in the more difficult regions of science they are our points of reference— often by their resemblances, and even by their differences, making plain what would otherwise be incomprehensible, and spreading a light over what would otherwise be obscure.

In the three divisions of material objects, which are so strikingly marked out for us by Nature, we find traits that are eminently characteristic. All our ideas of permanence and duration have a convenient representation in the solid crust of the earth, the mountains, and valleys, and shores of which retain their position and features unaltered for centuries together. But the air is the very type and emblem of variety, and the direct or indirect source of almost every motion we see. It scarce ever presents to us, twice in succession, the same appearance; for the winds that are continually traversing it are, to a proverb, inconstant, and the clouds that float in it exhibit every

possible colour and shape. It is, in reality, the grand origin or seat of all kinds of terrestrial motions. Storms in the sea are the consequences of storms in the air, and even the flowing of rivers is the result of changes that have transpired in the atmosphere.

But the interest connected with it is far from ending here. The atmosphere is the birthplace of all those numberless tribes of creation which constitute the vegetable and animal world. It is of materials obtained from it that plants form their different structures, and therefore from it that all animals indirectly derive their food. It is the nourisher and supporter of life, and in those processes of decay which are continually taking place during the existence of all animals, and which after death totally resolve their bodies into other forms, the air receives the products of those putrefactive changes, and stores them up for future use. And it is one of the most splendid discoveries of our times, that these very products which arise from the destruction of animals are those which are used to support the life and develop the parts of plants. They pass, therefore, in a continual circle-now belonging to the vegetable, and now to the animal world; they come from the air, and to it they again are restored.

It is not, therefore, the beautiful blue colour which the air possesses, and which people commonly call the sky, or the points of light which seem to be in it at night, or the moving clouds which overshadow it and give it such varied and fantastic appearances, or even those more imposing relations which bring it in connection with the events of life and death, which alone invest it with a peculiar claim on the attention of the student. Connected as it is with the commonest of every-day facts, it furnishes us with some of our most appropriate illustrations-those simple facts of reference of which I have already spoken, and to which we involuntarily turn when we come to investigate the more difficult natural phenomena.

Astronomical considerations show that the atmosphere does not extend to an indefinite region, but surrounds the earth on all sides to an altitude of about fifty miles. Compared with the mass of the earth its volume is quite insignificant; for as it is nearly four thousand miles from the surface to the centre of the earth, the whole depth of the atmosphere is only about oneeightieth part of that distance. Upon a twelve-inch globe, if we were to place a representation of the atmosphere, it would have to be less than the tenth of an inch thick.

Seen in small masses, atmospheric air is quite colourless and perfectly transparent. Compared with water and solid substances, it is very light. Its parts move among one another with the utmost facility. Chemists have proved that it is not, as the ancients supposed, an elementary body, but a mixture of many other substances. It is enough at present for us to know that its leading constituents are two gases, which exist in it in fixed quantities-they are oxygen and nitrogen-but other essential ingredients are present in a less proportion, such as carbonic acid gas, and the vapour of water.

Atmospheric air is taken by natural philosophers as the type of all gaseous bodies, because it possesses their general properties in the utmost perfection. Individual gases have their special peculiarities-some, for example, are yellow, some green, 'some purple, and some red.

The first striking property of atmospheric air which we encounter, is the facility with which the volume of a given quantity of it can be changed. It

is highly compressible, and perfectly elastic. A quantity of it tied tightly up in a bladder or india-rubber bag, is easily forced by the pressure of the hand into a less space. The material properties of the air, and its compressibility, are simultaneously illustrated by the experiment of the diving-bell, described under Fig. 6. A vessel forced with its mouth downward under water, permits the water to enter a little way, because the air contained in the vessel occupies less space under the pressure; but as soon as the vessel is again brought to the surface of the water, the air within it expands to its original bulk.

b

This ready compressibility and expansibility may be shown in many other ways. Thus, if we take a glass tube, Fig. 10, with a bulb, b, at its upper end, the lower end being open and dipping into a vessel of water, c, and having previously partially filled the tube with water to the height, a, it will be found, on touching the bulb with snow, or by pouring on it ether, or by cooling it in any manner, that the included air collapses into a less bulk. It is, therefore, compressible, and on warming the bulb with the palm of the hand, the air is at once dilated.

It is this quality of easy expansibility and compressibility which distinguishes all gaseous substances from solids and liquids. It is true the same property exists in them, but then it is to a far less degree. On the hypothesis that material bodies Fig. 10. are formed of particles which do not touch one another, but are maintained by attractive and repulsive forces at determinate distances, it would appear that, in a gas like atmospheric air, the repulsive quality predominates over the attractive; while in solids the attractive force is the most powerful, and in liquids the two are counterbalanced.

Again, as respects relative weight, the gases, as a tribe, are by far the lightest of bodies; and, indeed, it is among them that we find the lightest substance in Nature-hydrogen gas. They are, moreover, the only perfectly elastic substances that we know. Thus a quantity of atmospheric air compressed into a metal reservoir will regain its original volume the moment it has the opportunity, no matter how great may be the space of time since it was first shut up.

Under a relaxation of pressure this perfect elasticity displays itself in producing the expansion of gas. If a bladder, partially full of atmospheric air, be placed under an air-pump receiver, as the pressure is removed it dilates to its full extent, and might even be burst by the elastic force of the air confined within. The force with which this expansion takes place is very well displayed by putting the bladder in a frame as shown in Fig. 12, and loading it with heavy weights; as it expands by the spring of the air, it lifts up all the weights.

Fig. 11.

If we were to imagine a given volume of gas placed in an immense vacoum, or under such circumstances that no extraneous agency could act upon it, it is very clear that its expansion would be indefinitely great-the repulsive force of its own particles predominating over their attraction, and there being nothing to limit their retreat from one another. But when a gaseous mass surrounds a solid nucleus, the case is different-an expansion to a determinate and to a limited extent is the result. And these are the circumstances under which the earth and every planet surrounded by an elastic

atmosphere exists; for in the same way that our globe compels an unsupported body to fall to its surface, and makes projectiles, as bomb-shells and cannonshot-no matter what may have been the velocity with which they were urged-return to the ground, so the same attractive force restrains the indefinite expansion of the air and keeps the atmosphere, instead of diffusing away into empty space, imprisoned all round.

the

Besides this cause, gravitation to the earth, a second one, for the limited extent of the atmosphere, may also be assigned contraction, arising from cold. Observation has shown that, as we rise to greater altitudes in the air, the Fig. 12. cold continually increases; and gases, in common with all other forms of body, are condensed by cold. The attempt at unlimited expansion which the atmosphere, by reason of its gaseous constitution exerts, is therefore, kept in bounds by two causes-the attractive force of the earth and cold-and accordingly its altitude does not exceed fifty miles. From the circumstances that air is thus a compressible body, we might predict one of the leading facts respecting the constitution of the atmosphere -it is of unequal densities at different heights. Those portions of it which are down below have to bear the weight of the whole superincumbent mass; but this weight necessarily becomes less and less as we advance to regions which are higher and higher; for in those places, as there is less air to press, pressure must be less. And all this is verified by observation. The portions which rest on the ground are of the greatest density, and the density steadily diminishes as we rise. [These facts may be illustrated by the following simple experiment: Take four bags of wool and place them one above the other. It will then be evident that the lowest bag is compressed by the other three above it, while the bag that is placed upon the top is entirely uncompressed. Now, it is thus with the particles of which the air is composed, for we find that at the earth's surface the air is denser then on the top of a mountain.] Moreover a little consideration will assure us that there is a very simple relation between the pressure which the air exerts and its elastic force. Consider the condition of things in the air immediately around us if its elastic force were less, the weight of the superincumbent mass would crush it in; if greater, the pressure could no longer restain it, and it would expand. It follows, therefore, in the necessity of the case, that the elastic force of any gas is neither greater nor less, but precisely equal to the pressure which is upon it.

CHAPTER IV.

WEIGHT AND PRESSURE OF THE AIR.

Description of the Air-pump-Its Action-Limited Exhaustion-Fundamental fact that Air has Weight-Relative Weight of other Gases— Weight gives rise to Pressure-Experiments illustrating the Pressure of the Air.

In the year 1560, Otto Guericke, a German, invented the air-pump, and exhibited a number of very striking experiments before the Emperor Ferdinand III. This incident forms an epoch in physical science.

« PreviousContinue »