Crookes then worked on rarefied gases in vacuum tubes. He discussed his investigations in his 1878 Bakerian Lecture and his 1879 British Association Lecture, explaining the equipment he invented to carry out these experiments—the Crookes tube. “This instrument consists of a sealed glass tube from which nearly all the air has been removed and through the walls of which are passed two electrodes” (Infoplease). When Crookes introduced a small electrical charge between the two, he observed an unexplainable ray emitting from the negative electrode. Many of these rays miss the anode and strike instead the glass wall of the tube, causing it to exhibit fluorescence. The behavior of the rays indicates that they travel in straight lines and exert a pressure
