lost their virulence, while those forgotten in the open bottle in the incubator and exposed to the access of air had done so. Oxidation proved indeed to be one of the most general methods of artificially producing attenuated virus, to which method later on were added others—the effect of light, of chemicals, of passage through certain animals, etc.
And, of course, the last and crowning conclusion was that an ordinary, susceptible fowl that has undergone the injection with an attenuated culture becomes immune against a culture which kills other fowls; and that conclusion, in the particular circumstances under which Pasteur was working, proved to be true.
Pursuing the new line of research, Pasteur demonstrated that a protection similar to that obtained against smallpox and chicken cholera could be secured also against anthrax, a disease which, by the destruction it caused among sheep and cattle, was entailing heavy loss on the farmers of France. After a long series of experiments he prepared two specimens of virus, different in strength, but both weaker than the natural contagion, and worked out the proportions in which sheep, horses and cows could be safely injected first with the weakest virus and then with the virus of the somewhat greater strength, after which they became capable of withstanding the strongest anthrax infection.
In honor of Jenner, who was the first to discover the way of preparing a virus of a fixed strength safe to be used for the preventive treatment of men, Pasteur proposed that all such artificially bred, so to say, domesticated forms of microbes be called vaccines, while the word virus be reserved for a contagion growing in nature in a natural condition, or taken direct from an infected individual. The French distinguish between 'vaccin,' which is used as a generic term in Pasteur's sense, and 'vaccine,' which name they reserve for smallpox vaccinia lymph. The word 'vaccination' has been also extended to designate inoculation with artificially vaccinized virus, while the word 'inoculation' is used for the injection of a natural, not vaccinated virus, taken direct from a patient. The latter distinction is, however, not yet strictly maintained in English literature, nor in the subsequent pages of this paper.
Pasteur gave a memorable demonstration of the efficiency of his method of anthrax vaccination. At Pouilly-le-Fort, in the midst of an assemblage of scientists, delegates of agricultural societies, government officials, landlords, farmers and representatives of the press, he performed the following experiment: Sixty sheep were taken; ten of these were put aside, twenty-five were vaccinated with the two attenuated anthrax vaccines at an interval of twelve days, and twenty-five were left untouched. Twelve days afterward the two groups of twenty-five sheep were inoculated with virulent anthrax; and the result was that at the next visit the twenty-five unvaccinated and one vaccinated