From: J. Emsley, Nature's Building Blocks: an A-Z Guide to the
Elements, Oxford University Press, 2001, Chapter: Molybdenum, Section: Element of History, p. 263
"About this time (1781) Scheele discovered a simple and specific test
for molybdenum. When molybdenum was oxidized to molybdate (which is the metal in oxidation state VI),
it would form an intense blue colour on adding a reducing agent to the solution. Even minute amounts
of the metal could be detected this way and the depth of colour was a measure of the concentration.
The test was used for almost 200 years, despite the fact that chemists could not identify the agent
responsible for the colour. In 1996 the puzzle was solved by a group of German chemists at the
University of Bielefeld, who showed it to consist of a cyclical cluster made up of 154 molybdenum atoms
interlinked with oxygen atoms."