During this period his mind reverted constantly to the problem of the branched tubercle bacillus and the possibility which he had conceived of obtaining immunity to that disease by the introduction into the human economy of attenuated bacilli or of fluid extracts from the same, and he prosecuted original researches on this subject untiringly. On October 19, 1889, he published in the _Medical News_ of Philadelphia the results which he had then reached in producing immunity to that disease in the lower animals, and has constantly kept the profession informed of his investigations by means of addresses and contributions to medical and scientific journals. The two theses or propositions on which he based this theory of possible immunity to the tubercle bacillus are as follows: “First. It is possible that by a thorough filtering out of bacilli from tubercular material a filtrate might be obtained and attenuated so that by systematic inoculations a change might be produced in living animal tissues that would enable them to resist virulent tubercle bacilli. Second. To bring about a chemical or physical change in living tissues that would resist tubercular phthisis it is possible that inoculations with the bacillus would have to be made; yet, before this could be done, the power of the virulent bacilli would have to be diminished; otherwise the result would be most disastrous.” In the autumn of 1890, Doctor Dixon, inspired by the announcement of Professor Koch that he had succeeded in developing a substance which possessed the power of preventing the growth of the tubercle bacillus in the human economy, again visited Europe in order to assure himself of the accuracy of the reports and in the interest of humanity as well as of the various scientific and medical institutions with which he was connected, and had interviews with Koch, Virchow and Dubois Raymond. That Doctor Dixon has always recognized the claims of the community as a whole on members of his profession is sufficiently proven by the fact that he is officially connected with so large a number of scientific, philanthropic and educational institutions. He has been for eighteen years Executive Curator of the Philadelphia Academy of the Natural Sciences, and for fourteen years its President. For a number of years he was a member of the Board of Education of the city of Philadelphia an as Chairman of the Committee on Hygiene devoted much time to improving the sanitary condition of the schools. He is an officer of the Ludwick Institute, the Society for the Prevention of Tuberculosis in Pennsylvania, the Zoological Society of Philadelphia and the Grandom Institution, a member of the Council of the American Philosophical Society and of the Historical Society of Pennsylvania, a director of the Wistar Institute of Anatomy of the University of Pennsylvania, a Fellow of the College