The sixteenth volume of this unique work is composed of 25 letters concerning a range of subjects studied by Van Leeuwenhoek. Presented in the original Dutch with English translation, all of the letters were written from July 1707 to June 1712. The letters explore a host of disparate topics such as instruments to measure water; pulmonary diseases; experiments relating to the solution of gold and silver, salt crystals, and grains of sand; botanicals, such as duckweed and germination of orange pips; protozoa, blood, spermatozoa, and health and hygiene; the harmfulness of tea and coffee; and the benefits of cleaning teeth.