MLA

Gibbs, J. Willard (Josiah Willard), 1839-1903. Boltzmann, Ludwig (1844-1906) - Letter, signed, partially in autograph, to [Josiah Willard Gibbs]. Munchen.. Box 2, folder 37. 1892 May 12. https://collections.library.yale.edu/catalog/17296911.

APA, 6th edition

Gibbs, J. Willard (Josiah Willard), 1839-1903. (["Box 2, folder 37"])(1892 May 12)[Boltzmann, Ludwig (1844-1906) - Letter, signed, partially in autograph, to [Josiah Willard Gibbs]. Munchen.]. https://collections.library.yale.edu/catalog/17296911.