This volume (the 4th of a series of 19) contains 21 letters. Every volume in the series contains the texts in the original Dutch and an English translation. The great range of subjects studied by Van Leeuwenhoek is reflected in these letters: instruments to measure water; pulmonary diseases; experiments relating to the solution of gold and silver; salt crystals and grains of sand; botanical work, such as duckweed and germination of orange pips; descriptions on protozoa; blood; spermatozoa; and health and hygiene, for example and harmfulness of tea and coffee and the benefits of cleaning teeth