A widely used presumptive test that gives a variety of colour changes with a range of compounds. It is particularly useful for detecting opiate alkaloids and for amphetamines and methamphetamine. Marquis reagent is a mixture of methanal (formaldehyde) solution in water with sulphuric acid. Mescaline gives an orange colouration. With morphine, a violet colour is produced. Amphetamines give an orange-red colour and methamphetamine gives an orange colour. The two can be distinguished by the Simon test. The mechanism involves attack of the aldehyde an a substituted aromatic ring to form a carbocation. Further reaction forms a coloured dimer of the original molecule.