A fountain of Sicily which revealed if writings were authentic and genuine or not. The writings to be tested were thrown into the fountain, and if spurious they sank to the bottom. Oaths and promises were tried in the same way, after being written down. —Diodorus Siculus.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Acadine from Infoplease: