(8.) Fluid bodies are of two kinds, inelastic fluids, or liquids, and elastic fluids, or gases. Of the former of these classes, water is the most familiar example, and of the latter, air.

These two species of fluids are each distinguished by peculiar mechanical properties.

(9.) The constituent particles of a liquid are distinguished from those of solids by having little or no coherence; so that unless the mass be confined by the sides of the vessel which contains it, the particles will fall asunder by their gravity. A mass of liquid, therefore, unlike a solid, can never retain any particular form, but will accommodate itself to the form of the vessel in which it is placed. It will press against the bottom of the vessel which contains it with the whole force of its weight, and it will press against the sides with a force proportional to the depth of the particles in contact with the sides measured from the surface of the liquid above. This lateral pressure also distinguishes liquids from solids. Let

A

Fig. 3.

P

B

C

us take for illustration the case of a square or a cubical vessel, A B C D, fig. 3. If a solid body, such as a piece of lead, be cut to the shape of this vessel, so as to fit in it without pressing with any force against its sides, the mechanical effect which would be produced by it when placed in the vessel, would be merely a pressure upon the bottom, B C, the amount of which would be equal to the weight of the metallic mass. No pressure would be exerted against the sides; for the coherence of the particles of the solid maintaining them in their position, the removal of the sides would not subject the solid body contained in the vessel to any change.

Now let us suppose this solid mass of lead to be rendered liquid by being melted. The constituent particles will then be deprived of that cohesion by which they were held together; they will accordingly have a tendency to separate, and fall asunder by their gravity, and will only be prevented from actually doing so by the support afforded to them by the sides,

A B, DC, of the sure against the

vessel. They will therefore produce a pressides, which was not produced by the lead in its solid state. This pressure will vary at different depths: thus a part of the side of the vessel at p will receive a pressure proportional to the depth of the point P below the surface of the lead. If, for example, we take a square inch of the inner surface of the side of the vessel at P, it will sustain an outward pressure equal to the weight of a column of lead having a square inch for its base, and a height equal to P A. And, in like manner, every square inch of the sides of the vessel will sustain an outward pressure equal to the weight of a column of lead having a square inch for its base, and a height equal to the depth of the point below the surface of the lead.

(10.) We have here proceeded upon the supposition that no force acts on the upper surface A D of the lead. If any force presses A D downwards, that force would be transferred to the bottom by the lead, and would produce a pressure on the bottom B C equal to its own amount in addition to the weight of the lead; and if the lead were solid, this would be the only additional mechanical effect which such a force acting on the surface A D of the lead would produce. But if, on the other hand, the lead were liquified, then the force now adverted to, acting on the surface A D, would not only produce a pressure on the bottom в C, equal to its own amount in addition to the weight of the lead, but it would also produce a pressure against every part of the sides of the vessel, equal to that which it would produce upon an equal magnitude of the surface A D.

Thus if we suppose any mechanical cause producing a pressure on the surface A D amounting to ten pounds on each square inch, the effect which would be produced, if the lead were solid, would be an additional pressure on the base B C amounting to ten pounds per square inch. But if the lead were liquid, besides this pressure on each square inch of the base B C, there would likewise be a pressure of ten pounds on every square inch of the sides of the vessel.

All that has been here stated with respect to a square or a cubical vessel will be equally applicable to a vessel of any other form.

(11.) The second class of fluids are distinguished from liquids by the particles not merely being destitute of cohesion, but having a tendency directly the reverse, to repel each other, and fly asunder with more or less force. Thus if a vessel, such as that represented in fig. 3., were filled with a fluid of this kind, being open at the top, and not being restrained by any pressure incumbent upon it, the particles of the fluid would not rest in the vessel by their gravity, as those of the liquid would do; but they would, by their mutual repulsion, fly asunder, and rise out of the vessel, as smoke is seen to rise from a chimney, or steam from the spout of a kettle. Let us suppose, then, that the vessel in which an elastic fluid is contained is closed on every side by solid surfaces. In fact, let us imagine that the square or cubical vessel represented in fig. 3. is closed by a square lid at the top ▲ D, having contained in it an elastic fluid, such as atmospheric air.

If such a cover, or lid, had been placed upon a liquid, the cover would sustain no pressure from the fluid, nor would any mechanical effect be produced, save those already described in the case of the open vessel; but when the fluid contained in the vessel is elastic, as is the case with air, then the elasticity (by which name is expressed the tendency of the particles of the fluid to fly asunder) will produce peculiar mechanical effects, which have no existence whatever in the case of a liquid.

It is true that, supposing the fluid to be air or any other gas or vapour, a pressure will be produced upon the bottom B C of the vessel equivalent to the weight of such fluid, and lateral pressures will be produced on the different points of the sides by the weight of that part of the fluid which is above these points; but gases and vapours are bodies of such extreme levity, that these effects due to their weight are neglected in practice.

Putting, then, the weight of the air contained in the vessel out of the question, let us consider the effect of its elasticity. If the vessel, as already described, be supposed to contain atmospheric air in its ordinary state, the tendency of the constituent particles to fly asunder will be such as to pro

a pressure amounting to fifteen pounds; this pressure being, as already stated, quite independent of the weight of the air. In fact, this pressure would continue to exist if the air contained in the vessel actually ceased to have weight by being removed from the neighbourhood of the earth, which is the cause of its gravity.

(12.) Different gases are endowed with different degrees of elasticity, and the same gas may have its elasticity increased or diminished, either by varying the space within which it is confined, or by altering the temperature to which it is exposed.

If the space within which an elastic fluid is enclosed be enlarged, its elasticity is found to diminish in the same proportion. Thus if the air contained in the vessel A B C D (fig. 3.) be allowed to pass into a vessel of twice the magnitude, the elasticity of the particles will cause them to repel each other, so that the same quantity of air shall diffuse itself throughout the larger vessel, assuming double its former bulk. Under such circumstances, the pressure which it would exert upon the sides of the larger vessel would be only half that which it had exerted on the sides of the smaller vessel. on the other hand, it were forced into a vessel of half the magnitude of a BCD, as it might be, then its elasticity would be double, and it would press on the inner surface of that vessel with twice the force with which it pressed on that of the vessel A B C D.

If,

This power of swelling and contracting its dimensions according to the dimensions of the vessel in which it is confined, or to the force compressing it, is a quality which results immediately from elasticity, and is consequently one which is peculiar to the gases or elastic fluids, and does not at all appertain to liquids. If the liquid contained in the vessel A B C D were transferred to a vessel of twice the magnitude, it would only occupy half the capacity of that vessel, and it could not by any means be transferred, as we have supposed the air or gas to be, to a vessel of half the dimensions, since it is inelastic and incompressible.

(13.) The elasticity of gases is likewise varied by varying the temperature to which they are exposed; thus, in general,

if air or any other gas be augmented in temperature, it will likewise be increased in elasticity; and if, on the other hand, it be diminished in temperature, it will be likewise diminished in its elastic force. The more heated, therefore, any air or gas confined in a vessel becomes, the greater will be the force with which it will press on the inner surface of that vessel, and tend to burst it.

Ex

(14.) The same body may, by the agency of heat, be made to pass successively through the different states of solid, liquid, and gas, or vapour. The most familiar and obvious example of these successive transitions is presented by water. posed to a certain temperature, water can only exist as a solid; as the temperature is increased, the ice, or solid water, is liquefied; and by the continued application of heat, this water again undergoes a change, and assumes the form, and acquires the mechanical qualities, of air or gas: in such a state it is called STEAM.

This is a common property of all liquids. If they be exposed for a sufficient length of time to a sufficient degree of heat, they will always be converted into elastic fluids. These are usually distinguished from air and other permament gases, which never are known to exist in the liquid form, by the term vapour, by which, therefore, must be understood an elastic fluid which at common temperatures exists in the liquid or solid state; by steam is expressed the vapour of water; and by gases, those elastic fluids which like air are never known at least, under ordinary circumstances to exist in any other but the elastic form.

(15.) When a liquid is caused, by the application of heat, to take the form of an elastic fluid, or is evaporated, besides acquiring the property of elasticity, it always undergoes a considerable change of bulk. The amount of this change is different with different liquids, and even with the same liquid it varies with the circumstances under which the change is produced.

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(16.) When water is evaporated under ordinary circumstances, that is, when exposed to no other external pressure than that of the atmosphere, it increases its volume about