Diving bell with washed air, pneumatic. P.61.

Photograph: David Remes (2003)

Many of Joseph Priestley’s experiments with air of the mid 1770s were discussed at Lunar meetings, and Darwin’s Commonplace book reflects his interest in them:

There is a reason to believe from some of Dr Priestley’s experiments that air which has been breathed, by being washed in water becomes fit to breath again. Suppose a person was to put his head under a small diving bell, & by such a pump, as that described on the reverse page, could throw a stream continually in showers about the bell, would it not sufficiently purify the air? The water should be a running stream, & the pump takes it from a little distance. It might not be the same water again & again. This experiment with a candle & a syphon inverted under a cork might be easily tried.