Water contains a portion of air or other uncondensable gaseous matter, and when it is converted into steam, this air is mixed with it, and when the steam is condensed it is left in a gaseous state. If means were not taken to remove this air or gaseous matter from the condenser of a steam-engine, it would fill it and the cylinder, and obstruct their operation ; but, notwithstanding the ordinary means of removing it (by the air-pump), a certain quantity of it always remains in the condenser.

20 volumes of water absorb I volume of air.