Title page of Robert Boyle
's New Experiments
of 1660, in which he detailed how to perform the experiment.
In 1659, Robert Boyle commissioned the construction of an air pump, then described as a "pneumatic engine", which is known today as a vacuum pump. The air pump was invented by Otto von Guericke in 1650, though its cost deterred most contemporary scientists from constructing the apparatus. Boyle, the son of the Earl of Cork, had no such concerns—after its construction, he donated the initial 1659 model to the Royal Society and had a further two redesigned machines built for his personal use. Aside from Boyle's three pumps, there were probably no more than four others in existence during the 1660s: Christian Huygens had one in The Hague, Henry Power may have had one at Halifax, and there may have been pumps at Christ's College, Cambridge, and the Montmor Academy in Paris. Boyle's pump, which was largely designed to Boyle's specifications and constructed by Robert Hooke, was complicated, temperamental, and problematic to operate. Many demonstrations could only be performed with Hooke on hand, and Boyle frequently left critical public displays solely to Hooke—whose dramatic flair matched his technical skill.
Despite the operational and maintenance obstacles, construction of the pump enabled Boyle to conduct a great many experiments on the properties of air, which he later detailed in his New Experiments Physico-Mechanicall, Touching the Spring of the Air, and its Effects, (Made, for the Most Part, in a New Pneumatical Engine). In the book, he described in great detail 43 experiments he conducted, on occasion assisted by Hooke, on the effect of air on various phenomena. Boyle tested the effects of "rarified" air on combustion, magnetism, sound, and barometers, and examined the effects of increased air pressure on various substances. He listed two experiments on living creatures: "Experiment 40," which tested the ability of insects to fly under reduced air pressure, and the dramatic "Experiment 41," which demonstrated the reliance of living creatures on air for their survival. In this attempt to discover something "about the account upon which Respiration is so necessary to the Animals, that Nature hath furnish'd with Lungs", Boyle conducted numerous trials during which he placed a large variety of different creatures, including birds, mice, eels, snails and flies, in the vessel of the pump and studied their reactions as the air was removed. Here, he describes an injured lark:
…the Bird for a while appear'd lively enough; but upon a greater Exsuction of the Air, she began manifestly to droop and appear sick, and very soon after was taken with as violent and irregular Convulsions, as are wont to be observ'd in Poultry, when their heads are wrung off: For the Bird threw her self over and over two or three times, and dyed with her Breast upward, her Head downwards, and her Neck awry.
By the time Wright painted his picture in 1768, air pumps were a relatively commonplace scientific instrument, and itinerant "lecturers in natural philosophy"—usually more showmen than scientists—often performed the "animal in the air pump experiment" as the centrepiece of their public demonstration. These were performed in town halls and other large buildings for a ticket-buying audience, or were booked by societies or for private showings in the homes of the well-off, the setting suggested in both of Wright's demonstration pieces. One of the most notable and respectable of the travelling lecturers was James Ferguson FRS, a Scottish astronomer and probable acquaintance of Joseph Wright (both were friends of John Whitehurst). Ferguson noted that a "lungs-glass" with a small air-filled bladder inside was often used in place of the animal, as using a living creature was "too shocking to every spectator who has the least degree of humanity".
The full moon in the picture is significant as meetings of the Lunar Circle (renamed the Lunar Society by 1775) were timed to make use of its light when travelling. Beacon St, Lichfield WS13 7AD, and is recognisable as the site of the painting. The eight-paned window is unchanged; the door position remains as depicted in the painting, though the architrave is missing. A full moon can be seen from this room at the same bearing and inclination as in the painting, just after midnight when the moon is full. Darwin's "philosophical feasts" that started with the afternoon meal often carried on throughout the night.
Wright met Darwin in the early 1760s, probably through their common connection of John Whitehurst, first consulting Darwin about ill health in 1767 when he stayed in the Darwin household for a week. The energy and vivacity of both Erasmus and Mary (Polly) Darwin impressed Wright. In the 1980s Eric Evans (National Gallery) suggested that Darwin is the figure in the left foreground who holds a watch. As this composed timekeeper is not consistent with Darwin's flamboyant character, it is more likely that this is Dr William Small. The attention to timekeeping fits with Dr Small's role as the social secretary for the Lunar Circle. Small returned from Virginia in 1764 and established his practice in Birmingham in 1765, consistent with this being a meeting in 1767. The profile and wig of this figure are consistent with a contemporary portrait of Small by Tilly Kettle.