Pasteur hastened to apply his discovery of the attenuation of the virus of chicken cholera to the virus of splenic fever. Here, however, he was met with a serious difficulty. The microbes of splenic fever, if left in the flask for forty-eight hours, developed bright spots, and gradually into these spots the bacilli themselves seemed to be absorbed. Pasteur found these spots were the spores or seeds of the microbes, and he also found that, while the bacilli could be killed easily in various ways, the spores possessed a much greater resistance. They could be dried, for example, and preserved in that state indefinitely. It was apparent that the oxygenation which attenuated the venom of the bacilli of chicken cholera was impossible with those of splenic fever if the bacilli of the latter disappeared within a week, leaving the spores behind. But Pasteur had discovered before this that, unless the temperature of a fowl were lowered artificially, inoculation with the microbes of splenic fever would not produce the disease. From this he argued that, as the heat of the fowl's body was sufficient to resist the contagion, the bacilli themselves must be extremely sensitive to variations in temperature. He tried the experiment and found, by lowering the temperature of the flasks containing the cultures, he could prevent the formation of the spores. He then attenuated the venom of the splenic bacilli as he had that of the fowl cholera, tried it on guinea-pigs, found they became sick and recovered; inoculated them with the bacilli of full strength, but with no result. Pursuing his experiments, he discovered that he could by using vaccine-attenuated bacilli, of unequal strength, cause any degree of sickness he pleased. In the early part of 1881 Pasteur agreed to hold a public exhibition of his vaccine for splenic fever, the animals to be supplied by the Society of Agriculture in Melun. The experiment was begun on May 5th. Pasteur inoculated twenty-four sheep, one goat, and six cows with six drops each of attenuated virus, and twelve days afterward he reinoculated them with a stronger virus. On May 31st he reinoculated the thirty-one animals with the strongest virus of splenic fever, and at the same time inoculated twenty-five sheep and four cows which had not been vaccinated as were the others. On June 2d over two hundred people assembled at the farm to see the result. The twenty-five sheep that had not been vaccinated all died before that evening. The non-vaccinated cows had intense fever and great swellings, and could scarcely stand up. On the other hand, the vaccinated sheep and cows were in full health and were feeding quietly. Pasteur had conquered splenic fever.