Pneumatic Trough

Pneumatic Trough is a piece of chemical apparatus devised by Priestley, by means of which gases can be collected in vessels for experiments or examination. It consists of a vessel of water, provided with a ledge or shelf at the depth of two or three inches from the top. The jars in which the gas is to be collected are filled with water, and placed with their mouths downward upon the shelf, which is kept a little under water, so as to prevent the entrance of air into the jars. When the edge of the jar is brought over the extremity of the tube carrying the gas the bubbles of gas rise through the water, collect in the upper part of the jar, and displace the liquid.