In the cultivation of the microbe of fowl cholera, Pasteur tried one of the cultivating liquids which he had previously made use of with most success--the water of yeast--that is to say, a decoction of yeast in water rendered clear by filtration and then sterilised by a temperature of over 100 degrees. The most diverse microscopic organisms find in this liquid suitable nourishment, particularly if it has been neutralised. When, for example, the bacterium of splenic fever is sown in the liquid, it assumes in a few hours a surprising development. Now, it is remarkable that this medium is quite unsuited to the life of the microbe of fowl cholera. Not only does it not develop, but the microbe perishes in this liquid in less than forty-eight hours. May we not connect this singular fact with that which is observed when a microscopic organism proves innocuous in an animal which has been inoculated with it? It is innocuous because it cannot develop itself in the body of the animal, or because, its development being arrested, it cannot attain the vital organs. The decoction extracted from the muscles of the fowl is the only medium which really suits the microbe of fowl cholera. It suffices to inoculate the fowl with the hundredth, even the thousandth, part of a drop of this mixture, to produce the disease and cause death. But here is a strange peculiarity. If guinea-pigs are inoculated with this little parasite they are hardly ever killed by it. Guinea-pigs of a certain age generally exhibit only a local lesion at the point of inoculation, which ends in an abscess more or less prominent. After opening spontaneously, the abscess closes again and heals, while the animal preserves its appetite and its appearance of health. These abscesses sometimes last several weeks. They are surrounded by a pyogenic membrane and filled with a creamy pus, in which the microbe swarms side by side with the pus globules. It is the life of the microbe inoculated under the skin which causes the abscess. The abscess, with the membrane which surrounds it, becomes for the little organism a sort of closed vessel, which it is even easy to tap without sacrificing the guinea-pig. The organism is mixed with the pus in a state of great purity, and although it is localised its virulence is extreme. When fowls are inoculated with the contents of the abscess they die rapidly, while the guinea-pig, which has furnished the virus, gets well without the least suffering. A curious instance this is of the local evolution of a very virulent microscopic organism, which produces neither internal disorders nor the death of the animal upon which it lives and multiplies, but which can carry death to other species inoculated with it. Fowls and rabbits living among the guinea-pigs suffering from these abscesses might in a moment be smitten and perish, while the health of the guinea-pigs remained unchanged. To produce this result it would suffice that a little of the discharge