History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c36.s7.23

c36.s7.23

The chemical reactions, involved in the above methods for the preparation of calomel, may be explained as follows: Alum, magnesium sulphate or ferrous sulphate, when heated, yield some sulphuric acid. This reacts with common salt to liberate hydrochloric acid. The latter undergoes aerial oxidation in contact with ferric oxide from the brick dust, *gairika*, clay, etc., acting as a catalyst, and sets free chlorine, which attacks the mercury, giving rise to calomel. Alumina from alum or from clay may also catalyse the reaction to a certain extent like the ferric oxide.