MLA

Gibbs, J. Willard (Josiah Willard), 1839-1903. Bumstead, Henry Andrews (1870-1920) - Typed letter, signed, to Edwin Bidwell Wilson, regarding Gibbs' interest in statistical mechanics. New Haven.. Box 2, folder 44. 1915 December 22. https://collections.library.yale.edu/catalog/17296913.

APA, 6th edition

Gibbs, J. Willard (Josiah Willard), 1839-1903. (["Box 2, folder 44"])(1915 December 22)[Bumstead, Henry Andrews (1870-1920) - Typed letter, signed, to Edwin Bidwell Wilson, regarding Gibbs' interest in statistical mechanics. New Haven.]. https://collections.library.yale.edu/catalog/17296913.