1. W. Crookes, Radiant matter: A Resume of the principal lectures and papers of Prof. William Crookes on the fourth state of matter (James W. Queen & Co, Philadelphia, PA, USA, 1881)
2. W. Crookes, On Radiant Matter, 1880,
http://www.electrotherapymuseum.com/Library/SirWilliamCrookes/index.html
3. I. Langmuir, J. Franklin Inst. CXCVI, 1 (1923)
4. I. Langmuir, in The Collected Works of Irving Langmuir, edited by C. Guy Suits (Pergamon Press, 1961), Vol. 5, pp. 1–10.
5. I. Langmuir, Phys. Rev. 33, 954 (1929)