The iodine clock reaction (also known as the Harcourt-Esson reaction or the Harcourt-Essen reaction) is a classical chemical clock demonstration experiment to display chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. Two colorless solutions are mixed and at first there is no visible reaction. After a short time delay, the liquid suddenly turns to a shade of dark blue. The iodine clock reaction exists in several variations. In some variations, the solution will repeatedly cycle from colorless to blue and back to colorless, until the reagents are depleted.
Read more about Iodine Clock Reaction: Hydrogen Peroxide Variation, Iodate Variation, Persulfate Variation, Chlorate Variation
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—Administration in the State of Sout, U.S. public relief program (1935-1943)
“What says the Clock in the Great Clock Tower?
And all alone comes riding there
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Because he had feathers instead of hair.
A slow low note and an iron bell.”
—William Butler Yeats (1865–1939)
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—Eleanor Robson Belmont (1878–1979)