If fowls are naturally impervious to the infection of splenic fever, there is a disastrous malady to which they are subject, and which is commonly called 'fowl cholera.' Pasteur thus describes the disorder:—'The bird which is attacked by this disease is without strength, staggering, the wings drooping. The ruffled feathers of the body give it the shape of a ball. An overpowering somnolence takes possession of it. If forced to open its eyes, it appears as if it were awakened out of a deep sleep. Very soon the eyelids close again, and generally death comes without the animal changing its place, or without any struggle, except at times a slight movement of the wings for a few seconds.' The examination after death reveals considerable internal disorders.

Here, again, the disease is produced by a microscopic organism. A veterinary surgeon of Alsace, M. Moritz by name, was the first who suspected the presence of microbes in this disease; a veterinary surgeon of Turin, M. Peroncito, depicted it in 1878; a professor of the veterinary school of Toulouse, M. Toussaint, recognised it, in his turn, in 1879, and sent to Pasteur the head of a cock which had died of the cholera. But, however skilful they were, these observers had not succeeded in deciding the question of parasitism. None of them had hit upon a suitable cultivating medium for the parasite, nor had they reared it in successive crops. This, however, is the only method of proving that the virulence belongs exclusively to a parasite.

It is absolutely necessary, in the study of maladies caused by microscopic organisms, to procure a liquid where the infectious parasite can grow and multiply without possible mixture of other organisms of different kinds. An infusion of the muscles of the fowl, neutralised by potash, and rendered sterile by a temperature of 110 to 115 degrees, has proved to be wonderfully appropriate to the culture of the microbe of fowl cholera. The facility of its multiplication in this medium is almost miraculous. In some hours the clearest infusion begins to grow turbid, and is found to be filled with a multitude of little organisms of an extreme tenuity slightly strangulated at their centres. These organisms have no movement of their own. In some days they change into a multitude of isolated specks, so diminished in volume that the liquid, which had been turbid to the extent of resembling milk, becomes again almost as clear as at first. The microbe here described belongs to a totally different group from that of the vibrios. It is ranged under the genus called 'micrococci.' 'It is in this group,' said Pasteur on one occasion, 'that the microbes of the viruses which are yet unknown will probably be one day found.'

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 from the abscess of a guinea-pig should get smeared over the food of the fowls and rabbits. An observer witnessing such deaths without apparent cause, and ignorant of this strange dependency, would no doubt be tempted to believe in the spontaneity of the disease. He would be far from supposing that the evil had originated in the guinea-pigs, which were all in good health. In the history of contagia what mysteries may some day be cleared up by even more simple solutions than this one!

When some drops of the liquid containing this microbe are placed on the food of fowls, the disease penetrates by the intestinal canal. There the little organism increases in such great abundance that inoculation with the excrements of the injected fowls produces death. It is thus easy to account for the mode of propagation of this very serious disease, which depopulates sometimes all the poultry yards in the country. The only means of arresting the contagion is to isolate, for a few days only, the fowls and the chickens, to remove the dung heaps, to wash the yard thoroughly, especially with water acidulated with a little sulphuric acid, or carbolised water with two grammes of acid to the litre. These liquids readily destroy the microbe, or at least suspend its development. Thus all causes of contagion disappear, because, during their isolation, the animals already smitten die. The action of the disease, in fact, is very rapid.

The repeated cultivation of the infectious microbe in the fowl infusion, passing always from one infusion to the next following, by sowing in the latter an infinitely small quantity, so to speak, of the virus—as much, for example, as may be retained on the point of a needle simply plunged into the cultivation—does not sensibly lessen the virulence of the microscopic organism. Its multiplication inside the bodies of fowls is quite as easy with the last as with the first culture. In short, whatever may be the number of the successive cultures of the microbe in the fowl infusion, the last culture is still very virulent. This proves the microbe to be the cause of the disease—a proof the same in kind as that which had already enabled Pasteur to show that splenic fever and septicÆmia are produced by specific microbes.

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.