[Letter] 1875 April 6 [to] [Dr. Henry] / A.W. Hofmann.

Chicago citation style
Hofmann, August Wilhelm von, 1818-1892. [Letter] 1875 April 6 [to] [Dr. Henry] / A.W. Hofmann.. 1875-04-06. Retrieved from the Digital Public Library of America, http://cdm.lib.lehigh.edu/cdm/ref/collection/letters/id/1200. (Accessed September 23, 2018.)
APA citation style
Hofmann, August Wilhelm von, 1818-1892, (1875-04-06) [Letter] 1875 April 6 [to] [Dr. Henry] / A.W. Hofmann.. Retrieved from the Digital Public Library of America, http://cdm.lib.lehigh.edu/cdm/ref/collection/letters/id/1200
MLA citation style
Hofmann, August Wilhelm von, 1818-1892. Retrieved from the Digital Public Library of America <http://cdm.lib.lehigh.edu/cdm/ref/collection/letters/id/1200>.
Note: These citations are programmatically generated and may be incomplete.