Pneumatic trough

A pneumatic trough is a piece of laboratory apparatus used for collecting gases, such as hydrogen, oxygen and nitrogen. Pneumatic troughs require a liquid such as water. Scientists also have used mercury in pneumatic troughs, but usually only for the collection of water-soluble gases. Health and safety issues surrounding mercury generally prohibit its use in modern-day pneumatic troughs.

Pneumatic trough, as invented by Stephen Hales

Four items are required for gas collection with a pneumatic trough:

• The trough itself, which is a large glass dish or a similar container
• A gas bottle (or bulb), to hold the gas collected
• A way to support the gas bottle or bulb, such as a Beehive shelf or a hanger (see picture)
• A liquid in the trough

The bottle is filled with water, inverted, and placed into the pneumatic trough already containing water. The outlet tube from the gas-generating apparatus is inserted into the opening of the bottle so that gas can bubble up through it, displacing the water within.