Humboldt's papers in the Berlin State Library contain numerous unpublished manuscripts, which can shed new light on Humboldt's geognostic hypotheses: Immediately after the American Journey he carried out chemical experiments on the composition of the atmosphere with Louis-Jacques Thénard and Joseph Gay-Lussac (“Absorption de l’oxygène par les terres humectées”; “Expériments sur l’air atmosphérique”, <ref xmlns="http://www.tei-c.org/ns/1.0" target="http://kalliope-verbund.info/DE-611-HS-1280521">Papers Large Box 8</ref>). These experiments can be seen as a continuation of his observations on air purity and the assumed connection with the spread of epidemic diseases (especially yellow fever), which Humboldt had recorded a few months earlier in New Spain and Cuba (“C'est mon cahier de la Havane intitulé sur la salubrité de l'air”, 28 fols., <ref xmlns="http://www.tei-c.org/ns/1.0" target="http://kalliope-verbund.info/DE-611-HS-2849282">Vol. 1 of the Krakow Papers</ref>).