"Why did the problem of air become visible..? Why were Priestley, Boyle and Black able to see the question clearly enough..? ...because they had new tools. The air pump designed by Otto von Guericke and Boyle (...in collaboration with his assistant Robert Hooke...) were essential to Priestley's lab in Leeds. ...In a way, the air pump had enabled the entire field of pneumatic chemistry in the seventeenth century... [T]he air pump allowed you to... create a vacuum, which behaved markedly differently... even though air and the absence of air were visually indistinguishable."