Like the bacillus of splenic fever, the microbe of the fowl cholera is an aerobic organism. It is cultivated in contact with the air, or in aerated liquids. At the same time, though it is entitled to be called an aerobic organism, it differs essentially in certain respects from the parasite of splenic fever. If splenic fever blood filled with filaments of the parasite be enclosed in a vessel protected from the air--say, in a tube closed at its two extremities--in a few days, eight or ten at the most, and much fewer in summer, the parasite disappears, or rather is reduced to fine amorphous granulations, and the blood loses all its virulence. If the same system of shutting out the air be employed with the blood of a fowl charged with the microbe of fowl cholera, this microbe will be preserved with its virulence for weeks, months, even years. Pasteur has been able to keep for three years tubes thus sealed, a drop of blood from which when cultivated in fowl infusion, sufficed to infect the birds in the poultry yard with cholera. And not only is the microbe preserved thus in the blood contained in the tube; the same occurs if fowl infusion be put into tubes and then sealed by the flame of a lamp. When, in course of time, such tubes lose their virulence, it is because the vitality of the organism is extinct. The moment the contents of the tube cease to be virulent, it is a sign that the contagium is dead. It is useless, then, to attempt to cultivate it: the microbe cannot be revived. Here, then, is a third virulent disease, also produced by a microscopic organism. The characteristics of fowl cholera are very different from those of splenic fever and acute septicemia, and these three microbes do not in the least resemble each other. But, glancing back over Pasteur's work, are not the diseases of silkworms, pébrine and flacherie, also virulent diseases? Thus, in so many things, through so many studies, the same connection holds good. Each discovery of Pasteur's is linked to those which precede it, and is the rigorous verification by experimental method of a preconceived idea. 'Nothing can be done,' said he one day, 'without preconceived ideas; only there must be the wisdom not to accept their deductions beyond what experiments confirm. Preconceived ideas, subjected to the severe control of experimentation, are the vivifying flame of scientific observation, whilst fixed ideas are its danger. Do you remember the fine saying of Bossuet? "The greatest sign of an ill-regulated mind is to believe things because you wish them to be so." To choose a road, to stop habitually and to ask whether you have not gone astray, that is the true method.' It is this method which conducted him in 1880 to that wonderful discovery, the attenuation of contagia. What certain of these contagia are, we have already seen. We shall now learn what they become in the hands of Pasteur. _THE ATTENUATED VIRUS, OR VACCINATION OF VIRULENT DISEASES._