the vessel to burst. If the vessel is not brittle, but | forty-hundredth parts of a time heavier than water. possessed of considerable tenacity, as a leaden water-If there be three figures, thousandths of parts of a time pipe, the rupture will seldom be observed during the are meant; if four figures, ten thousandth parts; and continuance of the frost while the water remains in a so on. Common air is sometimes taken as a standard solid state, but it readily appears when thaw takes place, with which to compare gases, being a more simple mode as the water is then forced out with a velocity corres- of comparing the relative weights of aerial substances. ponding to the vertical height of the column of water But all the solids and liquids are estimated with refe in the pipe. The fissures of rocks, too, are widened by rence to water as the standard. the freezing of the water which may happen to lodge in them before frost; and this process, therefore, is a powerful agent in the disintegration of rocks. Portions of steep banks, also, from a similar cause, tumble down after thaw; for the moisture in them expands when frozen, and thus rends them to pieces, which, however, during the frost, are bound together as by cement, and fall down whenever thaw dissolves the moisture.

Heat has a powerful effect in causing certain bodies to shrink and diminish in volume. This happens with those substances which do not liquefy, such as wood and clay. The contraction arises from the heat carrying off the watery particles from the bodies, and thus allowing the constituent atoms to come more closely together. As wood becomes drier, its fibres are sometimes split asunder, so as to emit loud cracking noises, which, in the case of household furniture, are ascribed by the ignorant to supernatural causes.

Heat is further treated of under the articles Chemistry, Pnuematics, and Meteorology.

ACCIDENTAL PROPERTIES OF MATTER.

Having shown how the beautiful and extensive variety of form in bodies-solid, liquid, gaseous, and the different modifications of them are to be traced to the operation of chiefly two great leading principles in nature, attraction and repulsion, we have now to mention the peculiar forms or characters which bodies assume from the influence of these and other causes, and which are usually classed under the term accidental properties of matter. The following are these properties: -Density, Porosity or Rarity, Compressibility, Elasticity, Dilatation, Hardness, Brittleness, Malleability, Ductility, and Tenacity.

Density signifies closeness of texture, or compactness. Bodies are most dense when in the solid state, less dense when in the condition of liquids, and least dense of all when gaseous or aëriform. In this manner the degree of density is in agreement with the closeness of the atoms to each other. The density of bodies may generally be altered by artificial means, as is afterwards mentioned. The metals, in particular, may have the quality of density increased by hammering, by which their pores are made smaller, and their constituent particles are brought nearer to each other.

The more dense in substance that a body is, it is the more heavy or weighty. In speaking of the density of different solid and liquid bodies, the term specific gravity is used to denote the comparison which is made. Thus, the specific gravity of a lump of lead is greater than an equal bulk of cork; or the specific gravity of water is greater than that of an equal quantity of spirituous fluid. For the sake of convenience, pure distilled water, at a temperature of 62°, has been established as a standard by which to compare the specific gravity or relative weights of bodies. Water, as the standard, is thus said to be 1. When, therefore, any body, bulk for bulk, is double the specific gravity of water, it is called 2, and so on to 3 and 4 times, up to 22 times, which is the specific gravity of platinum, the heaviest known substance. In almost every case of comparison there are fractional parts, and these are usually written in figures, according to the following arrangement: Fractional parts are divided into tens, hundreds, thousands, and so on. If, in addition to the figure expressing the main part of the specific gravity, there be one other figure, with a dot or point between them-thus 2.5-the additional figure signifies tenths, and the body is two times and five-tenth parts of a time more dense or heavy than water. If two figures occur thus, 10:40-hundredths are signified, and the body is ten times and

Any body of greater specific gravity than water, will sink on being thrown into water; but it will float on the surface, if its specific gravity be less than that of water. A body, such as a piece of wood, after floating a certain length of time on water, will imbibe such a quantity of liquid that its specific gravity will be gra dually increased, and in the course of time it may sink to the bottom.

Porosity is the quality opposite to density, and means that the substance to which it is applied is porous; that is, full of small pores or empty spaces between the particles, and that the body is comparatively light. The instances of porosity are numerous in every department of the material world, but those which are connected with animal and vegetable bodies are the most remarkable. Bone is a tissue of pores or cells, and, when seen through a microscope, may be said to re semble a honey-comb. Wood is also a tissue of cells or tubes. If the end of a cylinder of straight wood be im mersed in water, whilst the other is forcibly blown into, the air will be found to pass through the pores of the wood, and rise in bubbles through the water. When a gas is comparatively light, it is said to be rare, or to pos sess rarity.

By compressibility is meant that quality in virtue of which a body allows its volume to be diminished, without the quantity or mass of matter being diminished. It arises, of course, from the constituent particles being brought nearer to each other, and is effected in various ways. All bodies are less or more capable of being diminished in bulk, which is a conclusive proof of their porosity. Liquids are less easily compressed than solid bodies; nevertheless they, to a small extent, yield, and go into smaller bulk by great pressure. The water at the bottom of the sea, by being pressed down by the superincumbent water, is more dense or compact than it would be at the surface. Atmospheric air and gases are much more easily compressed than liquids, or even than many solids. Air may be compressed into a hundredth part of its ordinary volume. When at this state of compression, it has a great tendency to expand and burst the vessel in which it is confined.

Some bodies have the power of resuming their former volume or shape when the force which dimi nished it is withdrawn. This quality is termed elasti city. Steel is one of the most elastic of metallic bodies, but its elasticity is not nearly so great as that of Indiarubber, which, though twisted, drawn out, or compressed in different ways, always resumes its original form. The aëriform fluids, such as atmospheric air, and the gases, are all exceedingly elastic; and so are liquids, such as water, but to a smaller extent.

Dilatability is that quality of bodies by which they are enabled to be expanded or enlarged in their dimen. sions, without any addition being made to their substance. Hardness is the quality which is the opposite of softness, and does not depend so much on the density of the substance, as the force with which the particles of a body cohere, or keep their places. For instance, glass is less dense than most of the metals, but it is so hard that it is capable of scratching them. Some of the metals are capable of being made either hard or soft. Steel, when heated to a white heat, and then suddenly cooled, as by immersion in water, becomes harder than glass; and when cooled slowly, it becomes soft and flexible. Brittleness is that quality by which bodies are capable of being easily broken into irregular fragments, and it belongs chiefly to hard bodies. Iron, steel, brass, and copper, when heated and suddenly cooled, become brittle. Malleability is the quality by which bodies are capable of being extended by hammering. Some of

the heavy one. The light one, on coming to a state of rest, will perhaps fall harmlessly on the ground, while the other, by its momentum, will strike forcibly on the earth, or destroy any object which opposes it. Momentum is proportionate to the mass and velocity of bodies, and, by multiplying the weight by the number of feet moved over per second, we find that the momentum is the product. Thus, if a body of twelve ounces move with a velocity of twenty feet per second, its momentum is (twelve times twenty) two hundred and forty. In ordinary language, the term impetus is used to signify the violent tendency of a moving body to any point. Before entering upon a consideration of motion as produced by ordinary forces, it will be appropriate to describe the effects produced upon bodies when simply falling-that is, moving downwards towards the earth, when the supports which upheld them are withdrawn.

THE PHENOMENA OF FALLING BODIES-WEIGHT, Attraction, as already explained, is a force inherent in nature, by which particles and masses of matter are drawn towards each other. This force, it has also been stated, increases in proportion to the quantity of matter which the attracting body contains, and it also increases as the bodies approach each other. Further, it has been mentioned that this powerful and subtile quality in matter is the cause of the falling or drawing of bodies downwards towards the earth, and thus produces what is termed weight or gravity. Gravity, then, is simply the tendency which any substance has to press downwards in obedience to the law of attraction, as exem-¦ plified in the phenomena of bodies falling from heights to the ground, when the supports which upheld them are removed.

All falling bodies tend directly towards the centre of the earth, in a straight line from the point where they are let fall. If, then, a body be let fall in any part of the world, the line of its direction will be perpendicular to the earth's centre. Consequently, two bodies falling on opposite sides of the earth, fall towards each other. Suppose any body to be disengaged from a height opposite to us, on the other side of the earth, its motion in respect to us would be upward, while the downward motion from where we stand, would be upward in respect to those who stand opposite to us, on the other side of the earth. In like manner, if the falling body be a quarter, instead of half the distance round the earth from us, its line of direction would be directly across or sidewise, that is, at right angles with the lines already supposed.

It will be obvious, therefore, that what we call up and down are merely relative terms, and that what is down in respect to us, is up in respect to those who live on the opposite side of the globe. Consequently, down every where means towards the centre of the earth, and up signifies from the centre of the earth. The velocity or rapidity of every falling body is uniformly accelerated, or increased, in its approach towards the earth, from whatever height it falls, if the resistance of the atmosphere be not reckoned. If a rock be rolled from the summit of a steep mountain, its motion is at first slow and gentle, but as it proceeds downwards, it moves with perpetually increased velocity, seeming to gather fresh speed every moment, until its force is sue that every obstacle is overcome; trees and rocks are dashed from its path, and its motion does not cease, until it has rolled to a great distance on the plain.

From the same principle, a person may leap from a chair without danger; but if he jump from the house. top, his velocity becomes so much increased, before he reaches the ground, as to endanger his life by the fall. │ It is found by experiment, that the motion of a fall. ing body is increased, or accelerated, in regular arithi. metical progression. In other words, in every second of time during its descent, it acquires an additional rate of speed, the rate regularly increasing by the accumu. lation of the preceding additions.

It is ascertained that a dense or compact body, when falling freely, passes through a space of 16 feet 1 inch during the first second of time. Leaving out the odd inch for the sake of even numbers, we find that the space fallen through in a given time is determined by the following arithmetical computation.

Ascertain the number of seconds which a body occu pies in falling. Take the square of that number (that is, the number multiplied by itself), and multiply the square by 16, which is the number of feet fallent during the first second, and the result is the amount of fe which the body altogether falls. For example, if a bai. occupy 3 seconds in falling, we take the square of 3, which is 9; then we multiply 9 by 16, which gives 144 as the result, and that is the number of feet faller. Again, if we find that the bail occupies 4 seconds iu fal.ing, we take the square of 4, which is 16, and multiplying 16 by 16, the result is 256, which is the number of feet fallen. And so on, always following the same ru.d of computation.

It is not always easy, by the above mode of calcula tion, to arrive at a correct result as to the height fallen by bodies, and all that can be expected is an approxmation to a true result. This arises from bodies being of different bulks, and receiving different degrees of opposition from the atmosphere in their descent. It is a common supposition that large and heavy bodies fail more quickly than small and light ones. This opinion, which was maintained even by philosophers, until Galleo rectified the mistake, perhaps originates in the error of confounding momentum with velocity. Be this as it may, it is now an ascertained truth in science, that ad bodies, of whatever density, fall with the same velocity. Thus, a ball containing a pound of lead falls with the same velocity as a ball containing an ounce. Tha equality in the rate of falling is, however, disturbed ly the quality of figure and bulk of bodies. A solid bai of gold will fall more quickly than the same quantity el gold beat out into a thin leaf, because in the case of th leaf the resistance from the atmosphere on a large su face impedes the descent. Thus the atmosphere pre vents bulky and porous substances from falling with the same velocity as those which are compact.

If the atmosphere were removed, all bodies, whether light or heavy, large or small, would descend with the same velocity. This fact is ascertained by experiments performed with the air-pump.

When a piece of coin, for instance a guinea, and a feather, are let fall at the same instant of time, from a hook which has held them at the top of the exhausted receiver of an air-pump, they are observed to fall at an equal rate, and to strike the bottom at the same moment. Hence it is demonstrated, that were it not for the resistance of the atmosphere, a bag full f feathers, and one of coins, would fall from a given height with the same velocity, and in the same space of tinie. It has been stated that the attraction of gravitatio increases in proportion to the quantity of matter whica the attracting body contains. Thus, the mass of our planet, the earth, exerts a force of attraction whi`n produces the phenomena of weight, and the falling bodies with a certain velocity.

The same principle of increased velocity in bodies as they descend from a height, is illustrated by pouring treacle, honey, or any thick syrup, from an elevated vessel. The bulky stream, which is perhaps two inches in diameter where it leaves the vessel, is reduced to the size of a straw or thread on reaching its destina- In consequence of the different size and density of tion; but what it wants in bulk is made up in velocity, the sun and planetary bodies, attraction is much strong r for the small thread-like stream at the bottom will fill in some of them than others, and consequently t a vessel just as soon as the large and slow moving stream, weight of bodies differs in each. On the surface of the at the ouflet; the velocity is indeed so great, that the sun, our pound weight would weigh upwards of 27 stream has not time to sink at once into the mass below, ' pounds, and a body would fall upon it 434 feet the first but falls in overlaying folds. i second. On the surface of Jupiter, our pound would