Anthonie Heinsius
Heinsius was a lawyer in Delft and member of the Veertigraad whose talents led him to become the city's pensionary (chief administrative officer). He would have regularly interacted with Leeuwenhoek when they worked together in Delft’s city hall from 1670 to 1679, when city secretary Heinsius was appointed pensionary for Delft in the States of Holland and began his long diplomatic career. This early part of Heinsius's life is discussed in detail in Het verstand komt met het ambt, referenced below.
The same year that Leeuwenhoek was appointed city inspector, 1679, Heinsius left Delft when he became pensionary for Delft in the States of Holland. In 1682 stadholder William III of Orange appointed Heinsius special negotiator to France. The mission was a failure but he made a favourable impression on William III.
Leeuwenhoek wrote six letters to Heinsius in 1683, when he was beginning his political career outside Delft. He wrote two more in 1685. In 1688 (Letter 65), Leeuwenhoek used him as a reference when writing to the Royal Society:
Some time ago when I was reporting to a certain Professor of Medicine my discovery relating to the circulation of the blood, this Gentleman told me that, when people were discussing my observations, and referring to them in confirmation of certain Matters, the response frequently was: are we to believe it just because Leeuwenhoek says so; what certainty do we have about it. For which reason that Gentleman warned me, and said that I would do well to produce an attestation of a few prominent persons who might have been eye-witnesses to these my discoveries, in order that I might suffer less contradiction on such related matters.
It is quite true that, from special considerations, I have not mentioned hitherto (in my letters) anybody by name, amongst those who, together with me, have seen with their own eyes some of the most remarkable things with the aid of my microscopes; but I only said in general that I had demonstrated the same to some Gentlemen with sound knowledge and judgment, Lovers of Natural Science.
But since I now hear that more credence will be given to what I say if I proceed to specify those who have partly seen the aforementioned circulation or course of the blood, concerning that about which I have now written and communicated to Your Honours, I will make no difficulty in mentioning, out of many, those names as I believe will merit the greatest confidence, such as Mr. Cornelis Schravesande, M.D., Full Lecturer in Anatomy and Surgery, Councillor and One-Time Magistrate. Mr. Cornelis Vallensis, LL.D., also Councillor and One-Time Magistrate. Mr. Antoni Heinsius, LL.D., Councillor and Pensionary of this City, and formerly Envoy Extraordinary to His Royal Majesty of France, and recently Commissary of this State to the Court of His Royal Majesty of England.
These Gentlemen, to whom I am accustomed to communicate many of my discoveries, I have shown, among other things, the veritable circulation of the blood, as distinctly as if we were looking with our naked eye at the movement of the water (in a running river).
Within a decade, Heinsius had become, somewhat reluctantly given the grisly history of the office, Grand Pensionary of the whole province of Holland. In that job until his death, he played a crucial role guiding the Republic and William III, prince of Orange, by then king of England, through wars with France and Spain. Leeuwenhoek wrote eight more letters to Heinsius at the end of the century.
A decade after that, toward the end of the Nine Year's War against France, Leeuwenhoek again began addressing letters to Heinsius, by then one of the most powerful diplomats in Europe. One letter 1695, his first to Heinsius in seven years, had a final paragraph that was crossed out for publication in the printer's working copy of the letter preserved at the University of Amsterdam. Leeuwenhoek wrote:
I should not have ventured to importune Your Honour with these discoveries of mine if it were not for the fact that Your Honour had assured me previously that he would find time, in spite of his occupations, to peruse my observations. Concluding, I remain, after presentation of my humble services ...
Both men are buried in the Oude Kerk, and Leeuwenhoek undoubtedly attended the funeral of his old friend in 1720.
Leeuwenhoek began the correspondence with Letter L-126 of 20 May 1683 to Heinsius, which is a short list of topics that he would write about in Letter L-128 of 16 July 1683 to Christopher Wren, among other things, frog sperm, the circulation of the blood, and digestion.
Letter L-127 is Heinsius’s first known letter to Leeuwenhoek, a reply to Letter L-126. The following week, Leeuwenhoek replied to Heinsius with Letter L-129 of 22 July 1683, with which he enclosed a copy of Letter L-128 to Christopher Wren. The follow-up Letter L-132 of 2 September 1683 is a note to Heinsius in Paris asking for Heinsius’s opinions about the observations in Letter L-128.
Heinsius’s next letter to Leeuwenhoek is Letter L-133 of 10 September 1683, in this volume, acknowledging receipt of Letter L-129, to which Leeuwenhoek replied with Letter L-134 of 16 September 1683 asking whether Heinsius would like to see a copy of Leeuwenhoek’s next letter to the Royal Society, about living organisms in human mouths and the structure of the skin. Leeuwenhoek's Letter L-136 of 30 September 1683 to Heinsius is a cover letter accompanying a copy of that letter, Letter L-135 of 17 September 1683 to Francis Aston.
In addition to Letter L-127, another four letters from Heinsius to L. are known: Letter L-139 of 8 October 1683, Letter L-161 of 3 August 1685, and Letter L-163 of 31 August 1685. Leeuwenhoek replied promptly to each of them. Heinsius’s final letter is thirty years later, Letter L-515 of 28 February 1715 thanking L. for his letters and praising his importance, to which Leeuwenhoek replied with his final letter, Letter L-526 of 25 February 1716. In those years between, L. wrote an additional 20 letters to Heinsius, about half of them with scientific observations. The others contain notes for letters and copies of letters to others. There is no known reply from Heinsius to any of them, suggesting some lost letters.
