1. Hooke, ” Micrographia”, Observation II (1665).
2. Herschel, quoted by Rayleigh, Proc. Roy. Inst., 16, 563 (1901).
3. Rayleigh, Proc. Roy. Inst., 16, 563 (1901).
4. Beilby, ” Aggregation and Flow of Solids”, Macmillan (1921).
5. Benedicks, Rev. Met., 19, 505 (1922).