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.
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 same strength 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 that the 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.