161. =Elasticity of the Air.=--All the phenomena cited in § 158, § 159, and § 160 exhibit the elasticity of the air. It is from this property that it is always disposed to expand. It will do so whenever pressure is taken from it, or when it can overcome pressure to which it is subjected. This property is most strikingly exhibited when the air is much condensed by pressure. And the greater the condensation the stronger is the expansive or elastic force. [Illustration: Fig. 109.] 162. =The Condenser.=--In Fig. 109 you have the plan of an instrument called the Condenser. In A B, a cylinder, moves the piston, P. Air is admitted to the cylinder at F, and into the receiver, V, at G. The valve at F prevents any air from escaping from the cylinder, and the valve at G prevents it from escaping from the receiver. The operation of the instrument is this: If the piston be pressed downward, the pressed air in the cylinder shuts the valve F and opens G, and so enters the receiver V. If now the piston be raised, air rushes in at F to fill the space in the cylinder. It can not come from V, because the valve G is shut by its pressure. By working the piston for some time you can get a body of air into V of very great density. You see that this instrument is the very opposite of the air-pump. In the receiver, V, you have condensed air, while in the receiver of the air-pump you have rarefied air. If you compare the two instruments you will see that the opposite results are owing to different arrangement of the valves. [Illustration: Fig. 110.] 163. =The Gasometer.=--Gas is distributed in pipes from the gasometer at the gas factory by the agency of the elasticity occasioned by condensation under pressure. The apparatus, Fig. 110 (p. 122), consists of a large round vessel, G, open below, and sunk in a larger vessel of water, _w_. We will suppose the vessel, G, to be full of water. Gas is introduced into it through the pipe, _p r_, the gasometer rising as it fills with the gas. P is a weight balancing the gasometer, and so permitting it to rise as the gas enters. The gasometer being filled, the gas is to be distributed. For this purpose weights are put upon the gasometer, so that the gas may be compressed. Under this pressure it by its elasticity seeks for more room, and obtains it by escaping through the pipe _o b c_. As the pressure on the gas needs to be regulated, there is sometimes a gauge, _h i_, attached, which shows the amount of the pressure. It is a bent tube with water in the bend. You see at once that the greater the pressure upon the gas the higher will the water be in the branch, _h_, of the gauge. [Illustration: Fig. 111.]