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(Warm-blooded). Class I.-MAMMALIA, or suck-giving animals, such as inan, monkey, bat,
whale, dog, &c.
Jointed Animals. Spiders and scorpions; insects, as beetles, crickets, dragon-flies, bees, butter
flies, and moths, bug, gnat, flea, &c. Crustacea, those which bave à covering similar to that of the water-flea,
the crab, lobster, &c. Barnacles and acorn-shells, worms and leeches.
Rayed Animals. Star-fishes and sea - urchins-jelly - fishes--zoophytes—internal worms--in
fusory animalcules-minute chambered shells—sponges.
In the preceding pages I have given four chapters upon animals ; and these may be regarded as representing the four great or primary groups, as above enumerated.
But the learner should ever bear in mind that classification is not fixed and unchangeable, but that it must from time to time undergo a revision, in order that it may be in conformity with our increasing knowledge of the structure, development, and habits of animals, and the affinities which they bear to each other.
LESSONS ON PHYSICS.
PROPERTIES OF MATTER.
Form.-The word Form is susceptible of a variety of significations. It is desirable to include them all. In the sense of aggregation of particles, the most obvious divisions of Form are the solid, the fluid, and the
gaseous. It is probable that all substances are capable of being changed from any one of these forms to any other, by the application or the abstraction of heat. Water becomes steam by the former agency, and ice by the latter. Lead, which is usually seen as a solid, is easily melted, and when in that state rapidly escapes in the shape of vapour. On the other hand, many gases have, by the united influence of cold and pressure, been converted into liquids and solids. Another signification of the word Form makes it equivalent to shape,—as when the form of a body is said to be spherical, or that of a surface to be square.
The same word is also employed to designate the structure of a body. In this sense the form may be crystalline as glass, fibrous as wood, or amorphous as clay.
From what has been said, it will be evident that under this manifold interpretation of the word Form, may be arranged a wide catalogue of the secondary qualities of bodies, such as density or heaviness, hardness and softness, ductility, tenacity, malleability, elasticity, &c. We shall explain the meaning of these terms.
Density is the most changeable of all the properties of matter. It varies with the temperature and the pressure to which bodies are subject. No doubt the ordinary changes in the size of solid bodies are but of small amount. They are indeed so small, that they are apt to escape detection. An example of contraction by cooling, which may perhaps have been noticed, occurs in the process of fitting the iron rim or tire to a wheel. The tire is put on when hot. As it cools, the metal contracts, and thus holds the wood-work with a firm grasp. The word density does not in all cases explain itself. It seems to imply a greater compactness or accumulation of particles in a dense body, without reference to the nature of the particles themselves. Another word or phrase is consequently necessary for the purpose of comparing different classes of bodies with regard to their weight.
Specific gravity is the phrase used to indicate the relative weights of the same volume of different substances. A sovereign is heavier than a shilling, but it is smaller. The specific gravity of gold is accordingly greater than that of silver. A cubic foot of water is heavier than the same volume of ice, and hence the specific gravity of water is properly said to be greater than that of ice. But when it is asserted, on the same grounds, that the density of water is greater than that of ice, a little explanation is requisite to make the assertion intelligible.
There is another property of matter which appears at first sight to depend on density, but which has in reality no connexion with it, viz. :
Hardness.—Oak is harder than deal, and it is denser ; but whilst iron is harder than lead, it is not so dense ; its specific gravity is less.
Ductility.—A body which is capable of being drawn out, so as to have its length increased, is said to be ductile. This property is possessed in a greater or less degree by all the metals. Gold is the most ductile. An' ounce of gold can be drawn into wire several miles in length ; and by coating silver with gold and then drawing it out, a single ounce may be made to extend to a length as great as the distance of Edinburgh from London.
Tenacity. It is a remarkable fact, that although the density of the metal is somewhat diminished by drawing, yet the weight which it will sustain, as compared with its thickness, is increased. Thus, a bar of wrought-iron of one square inch in thickness will sustain thirty tons. But if it be drawn out into wire of onethirtieth of a square inch, instead of sustaining only one ton, as it should do were its tenacity unchanged, it will sustain two tons. Hence a suspension-bridge hung on chain cables is much stronger than a similar bridge hung on solid iron cables of the same weight.
Wire cables are, however, apt to break after long use, probably from the iron becoming crystallized by friction and vibration.
Malleability.—This term is used to express the capacity which a substance has for being beaten or rolled out. Any attempt to beat or roll out glass is unsuccessful. It is, therefore, not malleable. Soft clay and dough are perfectly malleable. As a general rule, the more ductile substances are the more malleable. There are, however, some exceptions. Iron is more ductile than copper, but not so malleable. The malleability of metals is commonly increased by heat, and accordingly the smith heats his iron to a white heat, and as it cools beats it out into horse-shoes, &c. The beating also renders the metal denser and harder.
The divisibility of matter is one of those subjects which teach us the limited range of our senses. Although we can have little hesitation in believing that the ultimate particles of which matter is composed are of a definite size, still no limit towards which our senses enable us to approach seems at all near to the final point, beyond which no further march is possible. Thin as is paper, it is a thousand times as thick as the thinnest gold leaf. And the latter is found to be perfectly free from interstices. We must therefore conclude that the particles of matter, although not infinitely divisible, have a divisibility far beyond the reach of our senses.
Elasticity is that quality of certain bodies which enables them to recover their figure when it has been temporarily destroyed by violent bending or bruising. A saw is very elastic, but a sheet of lead is hardly so at all. A ball of ivory, when it falls on the ground, becomes slightly flattened, and its endeavour to recover its shape causes it to recoil from the ground. A ball of lead retains the indentation it has received, and does not recoil. The perfection with which bodies recover their figure is a measure of their elasticity. India-rubber is highly elastic i
unbaked clay possesses hardly any elasticity at all.
We may define Force as that which tends to produce motion. The following are some of the more important mechanical forces.
1. Universal gravitation. The principle known as universal
grayitation is this, that every particle of matter in the universe attracts every other particle with a force which varies inversely as the square of the distance, i. e., when the distance is doubled, the force is reduced to one-fourth, when trebled, to one-ninth, and so
This law satisfactorily explains the motions of the heavenly bodies. And so confident have mathematicians become of its universality, that they are disposed to refer any phenomena which it has failed to embrace rather to their ignorance of fact than to the inadequacy of the law. When certain unexplainable irregularities were detocted in the most remote known planet, it was confidently asserted that there must exist some still more remote body, the faint light of which might possibly never affect mortal eyes, but which, by its attraction, was producing the observed irregularities in the movements of the other. With a true spirit of philosophic inquiry, elaborate calculations were undertaken to discover whereabouts the unknown planet might be situated. The result was, that on the telescopes of astronomers being directed to the part of the heavens which the calculations indicated, the planet Neptune was immediately discovered.
The principle of universal gravitation is then firmly established as the great law of the universe—the law which rules all matter. But there are certain elements in the constitution of bodies which exhibit small or temporary deviations from this law. These elements, of which we know nothing except through their effects, are called non-material. The definition of matter, indeed, may be, that it is subject to the law of gravitation ; or, speaking of terrestrial matter, that it has weight. Thus gold, water, and air are material, whilst heat, light, and magnetism are immaterial. Whether these last are themselves substances in any sense, or are mere affections of matter, is a question not yet definitively answered. The popular belief is, that there exists a fluid infinitely more subtle than air, and that this fluid pervades all space, and flows freely amongst the particles of material bodies. To the movements and arrangements of the atoms of this fluid are attributed the phenomena of light, heat, &c. When, therefore, a stick of sealing-wax, after having been briskly rubbed on the coat-sleeve, is found to attract a feather, or when the pole of one magnet is found to repel the corresponding pole of another, neither of these phenomena must be regarded as having anything to do with the law of universal gravitation.
The attraction of one body towards another is so small, that it