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. If, on the other hand, it were forced into a vessel of half the magnitude of A B C D, 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. 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, [Pg029] 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. (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. Exposed 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.