the organism of an individual susceptible to the influence of the original disease, protected that individual by conferring an artificial immunity.
To these known facts regarding small-pox was added that knowledge gained during years of observation by both medical and lay men, that one attack of a contagious or infectious disease protected the individual, as a rule, from a subsequent attack if exposed thereto in later years. Little or no perceptible alteration existed in the human organism, and yet it had acquired some singular quality that enabled it to resist the infection of that disease if ever again exposed thereto.
It is to-day believed by excellent authorities that the processes by which this immunity is obtained vary as widely as do the processes of disease themselves. Each of these infectious diseases produces in the blood certain poisonous substances; and as man can be, so to speak, familiarized with a poisonous drug by the administration of doses that gradually increase from one that is harmless to one that ordinarily would be at once fatal, so there is probably a similar Mithridatic transformation in the character of the fluids of the body that have once met and conquered the toxic principle of an infectious or contagious disease.
Working on this hypothesis scientists have been experimenting with the introduction into the animal organism of the poisonous substances called toxines, toxalbumins, leucomaïnes, or ptomaïnes, that are produced by the micro-organisms of the various diseases, in order to determine the possibility of preventing the susceptibility for acquiring these diseases.
The discovery of a method that would protect an individual from cholera would be of great usefulness. For in India, the home of that disease, the average annual mortality therefrom in the cities is 3·32, and in the country 1·52 per 1,000 living. The army statistics show that 2·49 per cent of the European soldiers are admitted to the hospital for cholera, while only 0·95 per cent of the native soldiers are admitted for the disease; but the mortality, 33·69 per cent for the former, 35·5 per cent for the latter, is almost equal. In the various epidemic manifestations of cholera in various parts of the world the mortality has often exceeded 50 per cent of those attacked. In 1884 and 1885 cholera was epidemic in southern Europe, and in Spain in the latter year the official report states that there were almost one hundred and twenty thousand deaths. There were fifty-one persons affected in each thousand living, and the mortality was 36 per cent. These statistics stimulated investigators to attempt to solve the problem of affording immunity to cholera.
In March of 1885 Dr. J. Ferran, living in a small town in Catalonia, sent a communication to the French Academy of Sci-
