Copies of three letters sent by Johannes Hevelius to Henry Oldenburg Includes copies of: 1. A letter from Petrus Mollerus of Konigsberg to an unknown recipient. Expresses his high esteem of the Institution of the Royal Society, and giving an account upon various enquiries sent to him. In French, dated 6 July 1667. 2. A letter from a Consul of Dantzick Michael Behm to Hevelius. Contains accounts of new materials for Telescopical glasses, the way of making Potashes, amber, salt-mines in Poland and Sal Gemmae [rock salt], a query about cold, some anatomical considerations and a description of an odd birth of female twins. In Latin, dated 15 May 1667. 3. A letter from Monsieur Andrea Hedione to Federico Lennero MD regarding monstrous twins, dated 3 February 1667. Subject: Chemistry / Alchemy Read to the Royal Society on 5 December 1667 3 February 1667-6 July 1667