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Porosity.

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experiment :-A long glass tube, A (fig. 1), is provided with a brass cup, m, at the top, and a brass foot made to screw on to the plate of an air-pump. The bottom of the cup consists of a thick piece of leather. After pouring mercury into the cup so as entirely to cover the leather, the air-pump is put in action, and a partial vacuum produced within the tube. By so doing a shower of mercury is at once produced within the tube, for the atmospheric pressure on the mercury forces that liquid through the pores of the leather. In the same manner water or mercury may be forced through the pores of wood, by replacing the leather in the above experiment by a disc of wood cut perpendicular to the fibres.

When a piece of chalk is thrown into water air-bubbles at once rise to the surface, in consequence of the air in the pores of the chalk being expelled by the water. The chalk will be found to be heavier after immersion than it was before, and from the increase of its weight the volume of its pores may be easily determined.

The porosity of gold was demonstrated by the celebrated Florentine experiment made in 1661. Some academicians at Florence, wishing to try whether water was compressible, filled a thin globe of gold with that liquid, and, after carefully closing the orifice hermetically, they exposed the globe to pressure with a view of altering its form, well knowing that any alteration in form must be accompanied by a diminution in volume. The consequence was, that the water forced its way through the pores of the gold, and stood on the outside of the globe like dew. This experiment has since been repeated with globes of other metals, and like results obtained.

The Florentine academicians had concluded from their experiments that liquids were incompressible; that is, could not be reduced in volume by pressure. This, however, is not the case, liquids are compressible, though to a very small extent. By cooling, the diminution in volume is far more considerable.

From these facts we conclude that the molecules of liquids may be brought nearer each other, and therefore that there are pores between them. The facility, moreover, with which liquids mix is a proof of their porosity.

10. Applications of porosity. The property of porosity is frequently utilised, more especially in the process of filtration. This consists in clarifying liquids by freeing them from particles of matter which they hold in suspension; as is done, for instance, with

river water, which is turbid owing to the earthy matter it carries along with it.

The

The apparatus used for this purpose are called filters, and are usually constructed of unsized paper, felt, charcoal, etc. pores of these substances are sufficiently large to allow liquids to pass, but small enough to arrest the particles held in suspension. Figure 2 represents a filtering fountain, one side of which is sup

It

posed to have been removed, so that its construction is visible. consists of a box about a yard high divided in the inside into two compartments by a porous stone, A. The water to be filtered is placed in the upper compartment, whence it slowly percolates through the pores of the stone into the lower one, leaving behind it the foreign substances. In one of the sides of the box is a tube a, which terminates in the lower compartment, and allows the air to escape in proportion as water enters.

Figure 3 represents a filter known as the strainer of Hippocrates. It is a conical felt bag suspended by three cords, into which is poured the turbid liquor; it slowly traverses the pores, while all the solid particles to which the turbidity is due remain behind on the filter. This method is well-adapted for clarifying syrups, jellies, and liqueurs.

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Compressibility.

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Layers of powdered wood charcoal are also used for filtration. A layer of sand or of broken glass produces the same effect. The limpidity of well-water is due to the filtration through strata of earth.

II. Compressibility. This is the property which bodies possess of being diminished in volume by pressure without undergoing any loss of weight. Being due to the approach of the molecules, it is both a consequence and a proof of porosity.

Compressibility, is very marked in sponge, caoutchouc, cork, pith, paper, cloth, etc. Their volume is considerably diminished by mere pressure between

the fingers. The compressibility of metals is proved by the impression which they receive from the die, in the process of coining. There is, in most cases, a limit beyond which, when the pressure is increased, bodies are fractured or reduced to powder.

The compressibility of liquids is so small as to have remained for a long time undetected: it may, however, be proved by experiment, as will be seen in the chapter on HYDROSTATICS.

The most compressible bodies are gases, which by pressure may be made to Occupy ten, twenty, or a hundred times less space than under ordinary circumstances. The great compressibility of gases may be demonstrated by means of a glass tube with very thick sides, closed at one end and provided with a tight-fitting solid piston (fig. 4). The enclosed air cannot escape, and yet when the handle of the piston is pressed it can be moved down to one-half to three-quarters the length of the tube; proving that the volume of

Fig. 4.

the air is reduced necessarily to half or a quarter what it was originally. Most gases when thus compressed exhibit a remarkable property to which we shall revert, that, namely, of liquefying, or passing from the gaseous to the liquid state.

12. Elasticity.-Elasticity is the property which bodies possess of resuming their original form or volume, when, after having been compressed, bent, twisted, or pulled, the force which altered them has ceased to act.

Four kinds of elasticity may be distinguished; the elasticity by pressure, as in the case of gases; the elasticity by flexure, observed in springs; the elasticity of torsion, which is produced in linen or cotton threads when they are untwisted; and, finally, the elasticity of tension, which is that of piano or violin strings when they are stretched.

Elasticity, of whatever kind, is the result of a molecular displacement. If the molecules have been approximated by pressure, the repulsive force of heat tends to separate them; if, on the contrary, they have been separated, molecular attraction tends to bring them near each other again. If a piece of whalebone be bent, the

Fig. 5.

molecules in the concave part being compressed repel each other; in the convex part, where they are separated, they tend to approach each other; both these actions tend, therefore, to straighten it as soon as it is free.

Gases and liquids are perfectly elastic; in other words, they regain exactly the same volume when the pressure becomes the same. Solid bodies present different degrees of elasticity, though none present the property in the same perfection as liquids and gases, and in all it varies according to the time during which the body has been exposed to pres

sure. Caoutchouc, ivory, glass, and marble possess considerable elasticity; lead, clay, and fats, scarcely any.

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Applications of Elasticity.

I I

There is a limit to the elasticity of solids, beyond which they either break or are incapable of regaining their original form and volume. In sprains, for instance, the elasticity of the tendons has been exceeded. In gases and liquids, on the contrary, no such limit can be reached; they always regain their original volume.

The elasticity of solids may be demonstrated by the following experiment :—On a slab of polished black marble thinly smeared with oil, an ivory ball is allowed to drop from gradually increasing heights. Each time it will rebound and rise to a height a little less than that from which it fell, after having formed on the layer of oil a circular impression which is larger the greater the height of the fall (fig. 5). From this we conclude that the ball was flattened each time, and that it rebounded in consequence of the reaction of its compressed molecules.

13. Applications of elasticity.-Numerous applications of elasticity may be given. It is owing to their elasticity that corks are used for closing bottles. Pushed into the neck by the exercise of a certain force they become compressed, and then their elasticity causing them to press against the sides, they completely close the neck.

Children's balls depend upon the elasticity of gas: they are made of caoutchouc, and are inflated by air; when they strike against the ground, or against a wall, their volume diminishes, and the air which they contain being suddenly compressed, expands, and, acting like a spring, makes the ball rebound. A similar application is met with in air-cushions. They are made of an air-tight material, and are also inflated by air; they are at once compressible and elastic, and form a very soft seat.

Air-guns are a further application. The breech in these is made of steel, and is hollow; air is compressed in it by means of an instrument called the compression-pump, and being suddenly liberated its expansive force is sufficient to expel the projectile.

The use of carriage, and of watch and clock, springs depends upon the elasticity of steel. In like manner the elasticity of wool, hair, feathers, is utilised in mattresses, pillows, and seats.

Lastly, it is owing to their elasticity that piano, guitar, or violin strings are capable of being put into a vibratory motion, which, as we shall prove, is the origin of the sounds which stringed instruments yield.