ON GASES.
Gases are bodies which, unlike solids, have no independent shape, and, unlike liquids, have no independent volume. Their molecules possess almost perfect mobility; they are conceived as darting about in all directions, and are continually tending to occupy a greater space. This property of gases is known by the namesexpansibility,tension, orelastic force, from which they are often calledelastic fluids.
Gases and liquids have several properties in common, and some in which they seem to differ are in reality only different degrees of the same property. Thus, in both, the particles are capable of moving; in gases with almost perfect freedom; in liquids not quite so freely, owing to a greater degree of viscosity. Both are compressible, though in very different degrees.
If a liquid and a gas both exist under the pressure of one atmosphere, and then the pressure be doubled, the water is compressed by about the1⁄20000part while the gas is compressed by one-half. In density there is a great difference; water, which is the type of liquids, is 770 times as heavy as air, the type of gaseous bodies, while under the pressure of one atmosphere. A spiral spring only shows elasticity when it is compressed; it loses its tension when it has returned to its primitive condition. A gas has no original volume; it is always elastic, or in other words, it is always striving to attain a greater volume; this tendency to indefinite expansion is the chief property by which gases are distinguished from liquids.
Fig. 337.
Fig. 337.
Matter assumes the solid, liquid, or gaseous form according to the relative strength of the cohesive and repulsive forcesexerted between their molecules. In liquids these forces balance; in gases repulsion preponderates.
By the aid of pressure and of low temperatures, the force of cohesion may be so far increased in many gases that they are readily converted into liquids, and we know now that with sufficient pressure and cold they may all be liquified. On the other hand, heat, which increases thevis vivaof the molecules, converts liquids, such as water, alcohol and ether or gas into the aëriform state in which they obey all the laws of gases. The aëriform state of liquids is known by the name ofvapor, while gases are bodies which, under ordinary temperature and pressure, remain in the aëriform state.
In describing exclusively the properties of gases, we shall, for obvious reasons, refer to atmospheric air as their type.
Expansibility of Gases.This property of gases, their tendency to assume continually a greater volume, is exhibited by means of the following experiment:—A bladder, closed by a stop-cock and about half full of air, is placed under the receiver of the air pump, Fig.337, and a vacuum is produced, on which the bladder immediately distends.
Fig. 338.
Fig. 338.
This arises from the fact that the molecules of air flying about in all directions press against the sides of the bladder. Under ordinary conditions, this internal pressure is counterbalanced by the air in the receiver, which exerts an equal and contrary pressure. But when this pressure is removed, by exhausting the receiver, the internal pressure becomes evident. When air is admitted into the receiver, the bladder resumes its original form.
The compressibility of gasesis readily shown by thepneumatic syringe, Fig.338. This consists of a stout glass tube closed at one end, and provided with a tight-fitting packed piston. When the rod of the piston is pressed down in the cube, the air becomes compressed into a smaller volume; but as soon as the force is removed the air regains its original volume, and the piston rises to its former position.
Weight of Gases.From their extreme fluidity and expansibility, gases seem to be uninfluenced by the force of gravity: they nevertheless possess weight like solids and liquids. To show this, a glass globe of 3 or 4 quarts’ capacity is taken, Fig.339, the neck of which is provided with a stop-cock, which hermetically closes it, and by which it can be screwed on the plate of the air-pump.
The globe is then exhausted, and its weight determined by means of a delicate balance. Air is now allowed to enter, and the globe again weighed. The weight in the second case will be found to be greater than before, and if the capacity of the vessel is known the increase will obviously be the weight of that volume of air.
When the atoms or particles which constitute a body are so balanced by a system of attractions and repulsions that they resist any force which tends to change the figure of the body, they will possess a property, known by the name of elasticity.Elasticity, therefore, is the property which causes a body to resume its shape after it has been compressed or expanded.
Fig. 339.
Fig. 339.
Pressure exerted by Gases.Gases exert on their own molecules, and on the sides of vessels which contain them, pressures which may be regarded from two points of view. First, we may neglect the weight of the gas; secondly, we may take account of its weight. If we neglect the weight of any gaseous mass at rest, and only consider its expansive force, it will be seen that the pressures due to this force act with the samestrength on all points, both of the mass itself and of the vessel in which it is contained.
It is a necessary consequence of the elasticity and fluidity of gases thatthe repulsive force between the molecules is the same at all points, and acts equally in all directions.
If we consider the weight of any gas, we shall see that it gives rise to pressures which obey the same laws as those produced by the weight of liquids. Let us imagine a cylinder, with its axis vertical, several miles high, closed at both ends and full of air. Let us consider any small portion of the air enclosed between two horizontal planes. This portion must sustain the weight of all the air above it, and transmit that weight to the air beneath it, and likewise to the curved surface of the cylinder which contains it, and at each point in a direction at right angles to the surface. Thus the pressure increases from the top of the column to the base; at any given layer it acts equally on equal surfaces, and at right angles to them, whether they are horizontal, vertical, or inclined.
The pressure acts on the sides of the vessel, and it is equal to the weight of a column of gas whose base is this surface, and whose height its distance from the summit of the column.The pressure is also independent of the shape and dimensions of the supposed cylinder, provided the height remain the same.
For a small quantity of gas the pressures due to its weight are quite insignificant, and may be neglected; but for large quantities, like the atmosphere, the pressures are considerable, and must be allowed for.
Diffusion of gases.—Liquids mixed together, gradually separate, and lie superimposed in the order of their densities, and the surfaces of the separation of the liquids are horizontal. But when gases are mixed, they present other conditions of equilibrium, as follows.
1.—A homogeneous and persistent mixture is formed rapidly, so that all parts of the same volume are composed of the same proportions of the mixed gases.
2.—In a mixture of gases, the pressure (or elastic force), exercised by each of the gases, is the same as it was when alone.
3.—The rapidity with which the diffusion takes place, varies with the specific gravity of the gases. The more widely two gases differ in density, the quicker the process of intermixture.
Evaporation.—This is the slow formation of vapor from the surface of a liquid. The elastic force of a vapor which saturates a space containing a gas (like air), is the same as in a vacuum. The principal causes which influence the amount and rapidity of evaporation are as follows.
1st.—Extent of a surface.As the evaporation takes place from the surface, an increase of surface evidently facilitates evaporation.
2d.—Temperature.Increasing the elastic force of vapor, has a most important influence on the rapidity of evaporation; therefore the temperature of ebullition marks the maximum point of evaporation.
3d.—The quantity of the same liquid already in the atmosphereexercises an important influence on evaporation. The atmosphere can absorb only a certain amount of vapor, and evaporation ceases entirely when the air is saturated, but it is greatest when free from vapor, that is perfectly dry.
4th.—Renewal of the air.If currents of air are continually removing the saturated atmosphere from above the surface of a liquid, evaporation takes place most rapidly, since new portions of air, capable of absorbing moisture, are presented to it. Evaporation is therefore more rapid in a breeze than in still air.
5th.—Pressure on the surface of the liquid influences evaporation, because of the resistance thus offered to the escape of the vapor. That is to say—water boils more freely in an open vessel than within a steam boiler under pressure. Hence, the necessity for having large steam disengaging surfaces to prevent priming or lifting of the water when the boiler is forced beyond its rated capacity.