Henry Oldenburg wrote to Leeuwenhoek encouraging him and asking about the kind of salt he was observing

Date: 
October 26, 1674

Oldenburg acknowledged receiving Leeuwenhoek's recent letters, relayed greetings from Robert Boyle, and again encouraged Leeuwenhoek to continue sending his observations. He also asked for clarification about what kind of salt Leeuwenhoek was observing.

Leeuwenhoek replied to this letter on Letter 8 of 1674-12-04 (AB 13).

Document: 

The manuscript is missing; this letter was never published.

Sources