Hans Sloane wrote Letter L-421 of sometime between March and July 1704 about sending a packet of Philosophical Transactions and encouraging further research
This letter is known only by reference in Leeuwenhoek’s reply.
In this letter, sent separately from a packet of seven numbers of Philosophical Transactions, Sloane writes to Leeuwenhoek that he is pleased with Leeuwenhoek’s recent discoveries and he requests that Leeuwenhoek continue his research.
Philosophical Transactions nos. 283-288 are found in vol. 23. No. 289, the first number in vol. 24, is dated 28 February 1704. Together, they contain seven letters by Leeuwenhoek to the Royal Society, Letter L-408 of 25 December 1702, Letter L-409 of 5 February 1703, Letter L-410 of 26 February 1703, Letter L-413 of 3 November 1703, Letter L-414 of 4 December 1703, Letter L-415 of 8 January 1704, and Letter L-416 of 1 February 1704.
Letter L-423 of 22 July 1704 to the Royal Society
I duly received the Philosophical Transactions Nos 283-289, with which you were so good as to present me, for which generous present I am highly grateful. And eight days later I received by the ordinary post a letter written by the secretary, Mr. Hans Sloane, from which I saw that you were pleased with my discoveries contained in some letters written to you; he added the request that I should continue my researches into nature.
