224 THE POPULAR SCIENCE MONTHLY. 



the organism of an individual susceptible to the influence of the 

 original disease, protected that individual by conferring an arti- 

 ficial 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 sin- 

 gular 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 pro- 

 cesses 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 experiment- 

 ing with the introduction into the animal organism of the poison- 

 ous substances called toxines, toxalbumins, leucoma'ines, or pto- 

 maines, 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 ad- 

 mitted 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 epi- 

 demic 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- 



