Air (Elasticity Of). Suppose the two tight-bottomed hollow cylinders a and b, in the annexed figure, to be tilled with air : if we fit a plug so tightly to the sides of both, that no air can pass between it and the sides of the cylinder, and then try to force down this plug by pressure on the stem, we shall find a resistance to its downward motion. The plug, or piston, as it is called, descends indeed, but with increasing resistance as it goes down ; and if the pressure be removed, it returns to its former position, suddenly and with force. We have thus demonstrated not only that the air is a material substance, offering resistance; but also, that it is an elastic substance, capable of compression to an indefinite extent, and of restoration to its former condition on the withdrawal of the pressure.