While working on the bacilli of splenic fever, Pasteur had isolated the bacillus of chicken cholera, had cultivated it and had inoculated chickens with it, developing the disease. He found that so long as the cultures were made from flask to flask within twenty-four hours, the virus of the disease, that is, the power of the bacilli to produce cholera in the fowls inoculated, remained the same and the fowl died. But he discovered that if a flask containing the bacilli were left exposed to the air for two weeks, and the fowls were then inoculated with bacilli from this flask, they became sick, but did not die. Following this up, he inoculated a hen that had recovered from a sickness so produced, with the bacilli in their strongest and most virulent form, and the hen showed no effect whatever. Then he took two hens, one fresh from the coop and the other well again after the sickness produced by the inoculation with the exposed bacilli, and inoculated both with the blood of a hen that was dying of chicken cholera. The first died, the second was affected. In other words, Pasteur had made the greatest discovery in physiology of this century. He had found it is possible to attenuate the virus of a virulent disease, and to use that virus so attenuated as a vaccine matter which will guard the animal vaccinated against the disease. He had taken Jenner's discovery, and proved it applied to other diseases besides small-pox. Pasteur's theory of the reason why any vaccine matter will have its prophylactic effect, is this: He believes there is in the blood of any animal subject to a disease caused by bacilli some substance which is necessary to the sustenance of those bacilli; and when the bacilli, having an attenuated virus, are introduced, they slowly consume all of this substance. The substance being one which nature creates very slowly, no subsequent introduction of the bacilli, however virulent, can produce the disease until such time shall have elapsed that a new supply of the substance shall have been secreted. In this way he accounts for the fact that vaccination will protect from small-pox for a more or less defined period of time.