Letter, from Antoni van Leeuwenhoek to the Royal Society, dated at Delft Signed autograph Observations of the colony-forming flagellate 'Anthophysa vegetans' from Delft canal water; observations of dissolving silver in aqua fortis (nitric acid and sulphuric acid) and gold in aqua regia (mixture of nitric acid and hydrochloric acid); observations of the forms in which gold is precipitated from the solution; reasons why there is little duckweed on the surface of water in cold weather Read to the Royal Society on 17 March 1703 10 red-chalk figures attached English translation see EL/L3/50 Transcribed and translated in 'Alle de brieven van Antoni van Leeuwenhoek', edited, illustrated and annotated by Committee of Dutch Scientists, vol 14, p 181 English translation printed in 'Philosophical Transactions', vol 23, no 286, p 1430 5 February 1703