Wrote Letter L-109 of 1680-08-09 to Robert Hooke to ask whether the Royal Society received previous letters and to promise further investigations into the formation of blood

August 9, 1680
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Text of the letter in the original Dutch and in English translation from Alle de Brieven. The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren

Another three months had passed since Leeuwenhoek's last letter to Hooke and Gale. Receiving no response, Leeuwenhoek wrote this letter about new observations he was making. Perhaps remembering the previous years when he went many months without a reply, he added:

Also having made some observations which greatly strengthen my opinion on this subject, I resolved not to send you these before I was assured that you had received my previous missives. I shall also be glad to hear how these observations and slight considerations (in case you have received them) please yourself and the Honourable Royal Society and all the Philosophers.

Birch's History has no record that the members of the Royal Society noted this letter at a meeting.