Russian chemical engineer (b. 1842; d. 1915), born Yakov Fyodorovich Gartung (Яков Фёдорович Гартунг); also known outside Russia as Jacob Hartung.
Gartung was the uncle of Tchaikovsky's sister-in-law Praskovya.
Correspondence with Tchaikovsky
2 letters from Tchaikovsky to Yakov Gartung have survived, dating from 1884 and 1886, both of which have been translated into English on this website: