A method of testing for the presence of a halogen, nitrogen, or sulphur in an organic compound. A sample is heated in a test tube with a pellet of sodium. The hot tube is dropped into pure water and the fragments ground up in a mortar. The presence of a halogen (now in the form of a sodium halide) is detected by precipitation with silver nitrate solution. Nitrogen is revealed by the formation of a precipitate of Prussian blue on heating part of the solution with iron(II) sulphate solution containing hydrochloric acid and a trace of iron(III) ions. Lead ethanoate or sodium nitroprusside gives a precipitate with any sulphur present. Lassaigne’s test was invented by Jean Louis Lassaigne (1800–59) in 1843.