Litmus test

The metaphorical or idiomatic use litmus test to denote a defintive test of worth or genuineness dates from 1950s, and is now often used in place of the acid test. Litmus paper, familiar today in chemistry lessons at schools, when dipped into liquids, to determine whether they are acidic (the paper turns red) or alkaline (the paper turns blue) was first discovered in the late 19th century. See also acid test and touchstone.