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 pregnant female were found dead, while twenty-four out of the twenty-five that had been vaccinated were perfectly well, and exhibited during the whole time they were kept under observation the same degree of health as the ten sheep that had been put aside for comparison. * * * * * An impetus was given by these discoveries to researches having for their object the protection of _men_ against infectious diseases. The most important of these researches was Pasteur’s own into the nature of hydrophobia and rabies, and the way of inoculating against that disease. This was followed a few years later by the preparation of a prophylactic against cholera.