The acquisition by the Cambridge Mineralogical Museum of a series of well-crystallized specimens of chalybite, raised within recent years from the mines in the neighbourhood of Camborne in Cornwall, afforded a favourable opportunity for an investigation of the chemical composition and optical characters of this substance. This seemed the more desirable since no analysis of the chalybite crystals from the Camborne district appears to have been published, and our knowledge of the optical characters of the substance is confined to some observations of Ortloff on material containing large quantities of manganese carbonate from Wolfsberg in the Harz.