This essay examines the technical evolution and scientific context of William Crookes's effort to achieve an absolute vacuum in the 1870s. Prior to late 1876, along with interrogation of the radiometer effect, the quest for perfect vacuum was a major motive of his research programme. At this time, no absolutely dependable method existed to determine exactly the pressures at extreme rarefactions. Crookes therefore employed changes in radiometric, viscous and electrical effects with changing pressure in order to monitor the progress of exhaustion. After late 1876, his research priorities shifted because he had reached a plateau of technical accomplishment in the effort to attain extreme vacua, and because observed effects in vacua—particularly electrical—assumed an importance in their own right, and as bases for elucidation and defence of his concept of a ‘fourth state of matter’ at very low pressures.