Some years ago, in this same place, I was honored to expound the result of my alkalimetric tests with which I believed [translation error corrected—Eds.] to have definitely proved the excessive alkalinity of the atmosphere in Havana. Perhaps some of the Academicians present remember the alleged relations I sought to point out between that fact and the development of yellow fever in Cuba. But much has been done since then, more exact data has been collected, and the etiology of yellow fever has been more methodically studied than in previous times. I became, therefore, convinced that any theory that attributes the origin or propagation of that disease to atmospheric, miasmatic or meteorological influences, to uncleanliness or to the neglect of general sanitary measures, is absolutely untenable. I have left my original beliefs behind; and by saying this here, I want to justify this change of mind by submitting to the judgement of my distinguished colleagues a new series of experimental studies I undertook in order to find out how yellow fever spreads.