'He that thoroughly understands the nature of ferments and fermentations,' said the physicist Robert Boyle, 'shall probably be much better able than he that ignores them, to give a fair account of divers phenomena of certain diseases (as well fevers as others), which will perhaps be never properly understood without an insight into the doctrine of fermentations.'

At all times, medical theories, more particularly those which concern the etiology of virulent diseases, have had to encounter the opposition of explanations invented to account for the phenomena of fermentation. When Pasteur in 1856 began his labours on these subjects, the ideas of Liebig were everywhere revived. Like the ferments, so the viruses and processes of disease were considered as the results of atomic motions proper to substances in course of molecular change, and able to communicate themselves to the diverse constituents of the living body.

The researches of Pasteur on the part played by microscopic organisms in fermentation, changed the course of these ideas. The ancient medical theory of parasites and living contagia was revived. A German Professor, Dr. Traube, in 1864, put forward, in one of his clinical lectures, a new doctrine of the ammoniacal fermentation of urine.

'For a long period,' he said, 'the mucus of the bladder was regarded as the agent of the alkaline decomposition of urine. It was supposed that, in consequence of the distension produced by the retention of the liquid, the irritated bladder produced a larger quantity of mucus, and this mucus was regarded as the ferment which brought about the decomposition of urea, by an innate chemical force. This opinion (which was that of Liebig) cannot hold its ground in presence of the researches of Pasteur. This investigator has demonstrated, in the most decisive manner, that alkaline fermentation, like alcoholic and acetic fermentation, is produced by living organisms, the pre-existence of which in the liquid is the sine qu non of the process.' And Dr. Traube, citing some facts which confirmed the doctrine of Pasteur, concluded thus: 'Notwithstanding the long retention of the urine, its alkaline fermentation is not produced by an increased secretion of mucus or of pus; it only begins to develop from the moment when the germs of vibrios find access to the bladder from without.

The opposite doctrines of Liebig and Pasteur are here brought into clear juxtaposition; and thus was their mutual and reciprocal influence established in dealing with the etiology of one of the most serious diseases of the bladder. So far back as 1862, Pasteur, in his memoir on spontaneous generation, had announced, contrary to all the notions then held, that whenever urine becomes ammoniacal, a little microscopic fungus is the cause of this alteration. Later on he established that in affections of the bladder ammoniacal urine was never found without the presence of this fungus; and in order to show how in these studies therapeutic application often runs hand in hand with scientific discovery, Pasteur, having proved, with his assistant, M. Joubert, that boracic acid is antagonistic to the development of the ammoniacal ferment, advised Dr. Guyon, Clinical Professor of Urinary Diseases in the Faculty of Paris, to combat the dangerous ammoniacal fermentation by injection of boracic acid into the bladder. The celebrated surgeon hastened to follow this advice, and with the most happy results. While attributing to Pasteur the honour of this discovery, M. Guyon, in one of his lectures, said:—

'Boracic acid has this immense advantage, that it can be applied in large doses—3 to 4 per cent.—without causing the slightest pain. It has therefore become, in our practice, the agent continually and successfully used for injections. I also have recourse to a solution of boracic acid to produce large evacuations after the operation of breaking up stones in the bladder (lithotrity). I never omit to use this antiseptic agent in operations where breaking up is required, and I never wash the bladders of lithotritised patients with any other substance. I have also had good results from copiously washing the bladders and the wounds of patients on whom lithotomy has been performed with boracic acid. I always finish the operation by prolonged irrigations with a solution of from 3 to 4 per cent.'

It was not only into France and Germany that Pasteur's ideas penetrated; in England, surgery borrowed from Pasteur's researches important therapeutic applications. In 1865 Dr. Lister began in Edinburgh the brilliant series of his triumphs in surgery by the application of his antiseptic method, now universally adopted. In the month of February 1874 in a letter which does honour to the sincerity and modesty of the great English surgeon, he wrote to Pasteur as follows:—

'It gives me pleasure to think that you will read with some interest what I have written about an organism which you were the first to study in your memoir on lactic fermentation. I do not know whether you read the 'British Medical Journal;' if so, you will from time to time have seen accounts of the antiseptic system which for the last nine years I have been trying to bring to perfection. Allow me to take this opportunity of sending you my most cordial thanks, for having, by your brilliant researches, demonstrated to me the truth of the germ theory of putrefaction, thus giving me the only principle which could lead to a happy end the antiseptic system.'

Pasteur followed with lively interest the movement of thought and the successful applications to which his labours had given rise. It was a realisation of the hopes he had ventured to entertain. Already, in 1860, he expressed the wish that he might be able to carry his researches far enough to prepare the way for a profound study of the origin of diseases. And, as he gradually advanced in the discovery of living ferments, he hoped more and more to arrive at the knowledge of the causes of contagious diseases.

Nevertheless, he hesitated long before definitely engaging himself in this direction. 'I am neither doctor nor surgeon,' he used to repeat with modest self-distrust. But the moment came when, notwithstanding all his scruples, he could no longer be content himself to play the part of a simple spectator of the labours started by his studies on fermentation, on spontaneous generation, and on the diseases of wines and beer. The hopes to which his methods gave rise, the eulogies of which they were the object, obliged him to go forward. In February 1876 Tyndall wrote to him thus:—

'In taking up your researches relating to infusorial organisms, I have had occasion to refresh my memory of your labours; they have revived in me all the admiration which I felt on first reading them. It is my intention to follow up these researches until I shall have dissipated every doubt that has been raised as to the unassailable exactitude of your conclusions.

'For the first time in the history of science we are able to entertain the sure and certain hope that, in relation to epidemic diseases, medicine will soon be delivered from empiricism, and placed upon a real scientific basis. When this great day shall come, humanity will recognise that it is to you the greatest part of its gratitude is due.'

Pasteur approached the study of viruses by seeking to penetrate into all the causes of the terrible malady called splenic fever (charbon, Germ. Milzbrand). Each year this disease decimates the flocks not only in France but in Spain, in Italy, in Russia, where it is called the Siberian plague, and in Egypt, where it is supposed to date back to the ten plagues of Moses. Hungary and Brazil pay it a formidable yearly tribute; and to come back to France, the losses have amounted in certain years to from fifteen to twenty millions of francs. For centuries the cause of this pest has eluded all research; and further, as the malady did not always exhibit the same symptoms, but varied according to the kind of animal that was smitten by it, the disease was supposed to vary with the species that was attacked by it. The splenic fever of the horse was distinct from that of the cow; the splenic fever of horse and cow were again different from that of the sheep. In the latter, splenic fever was called sang-de-rate; in the cow, it was maladie du sang; in the horse, splenic fever; in man, malignant pustule.

It was not until 1850 that trustworthy data were first collected regarding the nature of the malady, its identity with and difference from other maladies. From 1849 to 1852 a commission of the Medical Association of Eure-et-Loir made a great number of inoculations, applied other tests, and proved that the splenic fever of the sheep is communicable to other sheep, to the horse, to the cow, and to the rabbit; that the splenic fever of the horse is communicable to the horse and to the sheep; that the splenic fever of the cow is communicable to the sheep, to the horse, and to the rabbit. As for the malignant pustule in man, no doubt remained that it must arise from the same cause as splenic fever in animals. What class of men is it that the malignant pustule most frequently attacks? Shepherds, cowherds, cattle breeders, farm servants, dealers in hides, tanners, wool cleaners, knackers, butchers—all who derive their living from domestic animals. In handling contaminated subjects the slightest excoriation or scratch of the skin is sufficient to allow the virus to enter. When others besides the class that we have named become infected, it is because they live in the neighbourhood of herds smitten with splenic fever. There are also certain flies which transport the virus. Suppose one of these flies to have sucked the blood of an animal which has died of splenic fever, a person stung by that fly is forthwith inoculated with the virus.

At the very time (1850) when these first experiments were being made by the Medical Association of the Eure-et-Loir, Dr. Rayer, giving an account in the 'Bulletin de la SociÉtÉ de Biologie de Paris' of the researches he had made, with his colleague, Dr. Davaine, on the contagion of splenic fever, wrote:—'In the blood are found little thread-like bodies about twice the length of a blood corpuscle. These little bodies exhibit no spontaneous motion.'

This is the date of the first observation on the presence of little parasitic bodies in splenic fever, but, strange to say, no attention was paid to these minute filaments. Rayer and Davaine also paid no attention to them. This indifference lasted for thirteen years; it would have lasted longer still, if the parasitic origin of communicable diseases had not been brought before the mind by each new publication of Pasteur's. From 1857 to 1860 it will be remembered that he had demonstrated lactic fermentation, like alcoholic fermentation, to be the work of a living ferment; in 1861 he had discovered that the agent of butyric fermentation consisted of little moving thread-like bodies, of dimensions similar to those of the filaments discovered by Davaine and Rayer in the blood of splenic fever patients; in 1861 he had announced that no ammoniacal urine existed without the presence of a microscopic organism; in 1863 he had established that the bodies of animals in full health are sealed against the introduction of the germs of microscopic organisms; that blood drawn with sufficient precaution from the veins and the arteries, and urine taken direct from the bladder, could be exposed to the contact of pure air without putrefaction, and without the appearance of living thread-like organisms of any kind whatever, mobile or immobile. It was all these facts which in 1863 brought back the attention of Davaine, as he himself has acknowledged, to the observation which he had made in 1850.

'M. Pasteur,' said M. Davaine in a communication made to the Academy of Sciences, 'published some time ago a remarkable memoir on butyric fermentation, which consists of little cylindrical rods, possessing all the characteristics of vibrios or of bacteria. The thread-like corpuscles which in 1850 I saw in the blood of sheep attacked with sang-de-rate, having a great analogy with these vibrios, I was led to examine whether filiform corpuscles, analogous to or of the same kind as those which determined the butyric fermentation, would not, if introduced into the blood of an animal, equally act the part of a ferment. Thus would be easily explained the alteration, and the rapid infection of the mass of the blood, in an animal which had received accidentally or experimentally into its veins a certain number of these bacteria—that is to say, of this ferment.'

But two summers passed before M. Davaine was able to procure a sheep affected with the sang-de-rate. It was only in 1863 that he first recognised the constant presence of a parasite, in the blood of sheep and rabbits which had died from successive inoculations with blood taken after death or in the last hours of life. He further proved that the inoculated animal, in the blood of which no parasites were as yet visible with the microscope, had every appearance of health, and that in these conditions the blood could not communicate splenic fever.

'In the present state of science,' Davaine concluded, 'no one would think of going beyond these corpuscles to seek for the agent of the contagion. This agent is visible, palpable; it is an organised being, endowed with life, which is developed and propagated in the same manner as other living beings. By its presence, and its rapid multiplication in the blood, it without doubt produces in the constitution of this liquid, after the manner of ferments, modifications which speedily destroy the infected animal.' 'For a long time,' he repeated, 'physicians and naturalists have admitted theoretically that contagious diseases, serious epidemic fevers, the plague, &c., are caused by invisible animalculÆ, or by ferments, but I do not know that these views have ever been confirmed by any positive observations.'

A few months after the publication of the results obtained by Davaine, two professors of Val-de-GrÂce, MM. Jaillard and Leplat, sought to refute the preceding conclusions. After having inoculated rabbits and dogs with various putrefying liquids filled with vibrios, they could not cause the death of these animals. To bring about this result it was necessary to introduce into the blood of these dogs and rabbits several cubic centimeters of very putrid liquid. Again in this case, which only added another example to the experiments of Gaspard and Magendie upon the action of putrid liquids, they failed to generate any virulence in the blood. Davaine had no difficulty in showing that MM. Jaillard and Leplat's experiments were made under conditions totally different from his; that he, Davaine, had not made use of the vibrios or bacteria of unselected infusions, but of bacteria which had been found in the blood of sheep which had died from sang-de-rate.

Jaillard and Leplat returned to the charge, and this time with entirely new and unexpected experiments. They inoculated some rabbits, as Davaine desired, with the blood of a cow which had died of splenic fever. The rabbits died rapidly, but without showing before or after their death the least trace of bacteria. Other rabbits, inoculated with the blood of the first, perished in the same manner, but it was still impossible to discover any parasite in their blood. MM. Jaillard and Leplat offered Davaine some drops of this blood. Davaine, taking up the experiments of his opponents, confirmed the exactitude of the facts they had announced, but concluded by saying that these two professors had not employed true splenic fever blood, but the blood of a new disease, unknown up to that time, which Davaine proposed to call the cow disease.

'The blood which we used,' replied MM. Jaillard and Leplat, 'was furnished to us by the director of the knacker's establishment of Sours, near Chartres, and this director is undeniably competent as to the knowledge of splenic fever.'

Full of sincerity and conviction, MM. Jaillard and Leplat recommenced their experiments, using this time the blood of a sheep which had died of splenic fever, and which M. Boutet, the most experienced veterinary surgeon of the town of Chartres, had procured for them. Their results were the same as those obtained with the blood of the cow. Notwithstanding the replies of Davaine, which, however, added nothing to the facts already adduced on one side or the other, it was difficult to pronounce decidedly in such a debate. Unprejudiced minds received from these important discussions the impression that Jaillard and Leplat, in producing facts the exactitude of which were admitted by Dr. Davaine himself, had given a blow to the assertions of the latter, and that the subject required, in every case, new experimental studies.

In 1876, a German physician, Dr. Koch, took up the question. He confirmed the opinion of Davaine, but without in the least producing conviction, since he threw no light upon the facts adduced by MM. Jaillard and Leplat, of which, indeed, he did not even deign to speak. At the very same moment when the memoir of Koch appeared in Germany, the eminent physician Paul Bert came forward to corroborate the opinion of Jaillard and Leplat.

'I can,' said M. Paul Bert, 'destroy the bacteria in a drop of blood by compressed oxygen, inoculate with what remains, and reproduce the disease and death without any appearance of bacteria. Therefore, the bacteria are neither the cause nor the necessary effect of the disease of splenic fever. It is due to a virus.'

This was indeed the opinion of Jaillard and Leplat. Pasteur, in obedience to the necessity he felt to get at the fundamental truth of things, and also in his eager desire to discover some decisive proofs as to the etiology of this terrible disease, resolved in his turn to attack the subject.

Dr. Koch had stated in his memoir that the little filiform bodies, seen for the first time by Davaine in 1850, had two modes of reproduction—one by fission, which Davaine had observed, and another by bright corpuscles or spores. The existence of this latter mode of reproduction Pasteur had already discovered in 1865, reasserted and illustrated in 1870, as being common to the filaments of the butyric ferment, and to all the ferments of putrefaction. Was Dr. Koch ignorant of this important fact, or did he prefer by keeping silence to reserve to himself the advantage of apparent priority?

In order to solve the first difficulty which presented itself to his mind—that is to say, the question as to whether splenic fever was to be attributed to a substance, solid or liquid, associated or not associated with the filaments discovered by Davaine, or whether it depended exclusively upon the presence and the life of these filaments—Pasteur had recourse to the methods which for twenty years had served him as guides in his studies on the organisms of fermentation. These methods, delicate as they are, are very simple. When he wished, for example, to demonstrate that the microbe-ferment of the butyric fermentation was the very agent of decomposition, he prepared an artificial liquid formed of phosphates of potash, of magnesia, and of sulphate of ammonia, added to the solution of the fermentable matter, and in this medium he caused the microbe-ferments to be sown in a pure state. The microbe multiplied, and provoked fermentation. From this liquid he could pass to a second or third fermentable liquid composed in the same manner, and so on in succession. The butyric fermentation appeared successively in each. Since the year 1857 this method was supreme. In this particular research on the disease of splenic fever Pasteur proposed to isolate the microbe of the infected blood, to cultivate it in a state of purity in artificial liquids, and then to come back to the examination of its action on animals. But as, since his attack of paralysis in 1868, Pasteur had not recovered the use of his left hand, and consequently found it impossible to carry on a long series of experiments alone, he was obliged to seek for a courageous and devoted assistant. He found one in a former pupil of his at the École Normale, M. Joubert, now Professor of Physics at the CollÈge Rollin. If M. Joubert incurred the danger of these experiments on splenic fever, he also shared with Pasteur, in the Comptes-rendus of the Academy of Sciences, the honour of the researches and the triumph of the discoveries.

On April 30, 1877, Pasteur read to the Academy of Sciences, in his own name and in that of his fellow-worker, a note in which he demonstrated, this time in a completely unanswerable manner, that the bacilli called bacteria, bacterides, filaments, rods, in a word the bacilli discovered by Davaine and Rayer in 1850, constituted the only agent of the malady.

A little drop of splenic fever blood, sown in urine or in the water of yeast, previously sterilised—that is to say, rendered unputrescible by contact with air free from all suspended germs—produces in a few hours myriads of bacilli or of bacteria. A little drop of this first cultivation sown in a second flask containing the same liquid as the first and prepared with the same precautions as to sterility and purity, shows itself no less fertile. Finally, after ten or twenty similar cultures the parasite is evidently freed from the substances which the initial drop of blood might carry with it; yet, if a very small quantity of the last culture is injected under the skin of a rabbit or a sheep, it kills them in two or three days at most, with all the clinical symptoms of natural splenic fever.

It might be objected that the parasite was associated in the cultivating liquid with some dissolved substance that it had produced during its life and which acted as a poison. Pasteur accordingly transported some cultivating tubes into the cellars of the Observatory, where a temperature absolutely constant reigned, a circumstance which permits of the deposit of all the parasitic filaments at the bottom of the tubes. Inoculating afterwards both with the clear upper liquid and with the deposit at the bottom, he found that the latter alone produced disease and death. It is, then, the bacteria which cause splenic fever. The proof was given and no further doubt remained.

I.

Yes, splenic fever is no doubt produced by bacteria just as itch is produced by acaries and trichinosis by trichinÆ. The only difference is that the parasite of splenic fever can only be seen by means of a rather powerful microscope. Here, then, is a disease in the highest degree virulent, due in its first cause to the infinitely little. Pasteur laid hold of and isolated this terrible virus. It was in a microscopic parasite, and in it alone, that the virulence of splenic fever resided. A great scientific fact had been gained. A virus might consist not of amorphous matter, but of microscopic beings. The virulence was due to their life.

Liebig, and all the chemists and doctors who had accepted and maintained his doctrine, totally repudiated all vital action in fermentation as well as in contagious and infectious diseases. Dominated by their hypotheses, they allowed themselves to be deceived by false assimilations to facts of a purely chemical kind, which appeared to them to be connected with the phenomena of fermentation and virulence.

Liebig wrote, 'By the contact of the virus of small-pox the blood undergoes an alteration, in consequence of which its elements reproduce the virus, and this metamorphosis is not arrested until after the complete transformation of all the globules capable of decomposition.'

This vague theory of viruses was forced to give way before the multiplied experiments of Pasteur. But before occupying himself with further discoveries, although it had been irrefutably proved that the microscopic parasite was the true contagium, it was necessary to throw light upon the facts, mainly accurate, which had been announced by Jaillard and Leplat, and to bring them into harmony with the facts, not less certain, which had been advanced by Davaine. The rabbits which Jaillard and Leplat had inoculated with a drop of the blood of a cow or sheep stricken with splenic fever, died rapidly, and the blood of these rabbits was shown to be also virulent. It was sufficient to inoculate other rabbits with a very minute quantity to cause their death. But Jaillard and Leplat affirmed that the examination of that blood did not reveal the existence of any microscopic organisms. Paul Bert, on his part, had succeeded in destroying the bacteria by compressed oxygen, and yet the virulence had continued.

Were there, then, two kinds of virus? What escape was there from this darkness? A new light suddenly began to dawn. Pasteur had already some years previously demonstrated that the animal body is sealed against the introduction of lower organisms—that in the blood, the urine, the muscles, the liver, the spleen, the kidneys, the brain, the marrow, and the nerves, in a normal state, no germ is found, or particle of any kind, known or unknown, which could be transformed into bacteria, vibrios, monads, or microbes. The intestinal canal alone is filled with matters associated with a host of germs and living products in process of development, and in divers states of physiological action. Not only is its temperature favourable to the life of infusoria, but it receives incessantly matters charged with the germs of these microscopic organisms. To the upper portions of the canal the air still has access, so that even in the stomach aerobic microbes may be found, but in the lower parts of the intestinal canal oxygen is absent, and only anaerobic microbes can be developed there. Although the life exerted in the mucous surface of the intestines opposes itself to the passage of those little organisms into the interior of the body, this ceases to be the case after death. There is no longer any obstacle to arrest or prevent them from acting according to the respective laws of their evolution and of the decomposing influence which belongs to them. It is by anaerobic organisms, in fact, that the putrefaction of dead bodies is begun. They penetrate into the organs and into the blood as soon as this liquid is deprived of oxygen; and it is not long before this happens, the oxygen fixed in the globules being soon consumed. In the body of an animal which has died of splenic fever, putrefaction is still more rapid, because, through the action of the disease, the blood is already in a great degree deprived of oxygen at the time of death. Nothing is more striking than the rapid inflation and almost immediate putrefaction of animals which have succumbed to splenic fever. Of all the vibrios ready to pass from the intestinal canal into the network of mesenteric veins which surround the canal those which seem to take the foremost place are the septic vibrios. These specially merit the name of vibrios of putrefaction, from the very putrid gases which result from their action upon nitrogenous and sulphurous substances. The others diffuse themselves more or less slowly in the blood, but the septic vibrio takes almost immediate possession of the dead body. Already after twelve or fifteen hours, the blood of the diseased animal, which at the time of its death and during the first following hours contained exclusively the parasite of splenic fever, harbours at one and the same time both the bacillus of splenic fever and the septic vibrio. Then occur the very curious effects arising from the anaerobic nature of these vibrios, and their opposition to the bacillus of splenic fever, which is exclusively aerobic. Diffused in blood deprived of oxygen gas, the splenic bacillus soon perishes. In its place are to be found amorphous granulations deprived of all virulence. The septic anaerobic vibrio, on the contrary, finds itself after death in the most favourable conditions for its life and development. Not only does it penetrate into the blood by the deep mesenteric veins, but also into the liquids which ooze out of the abdomen and muscles.

From the antagonism existing between the physiological peculiarities of the splenic bacilli and the septic vibrio, it results that if, in order to inoculate an animal capable of contracting the fever, a drop of blood be taken from one that has just died of it, and if the operation is performed during the first few hours after death, it is certain to communicate to that animal splenic fever, and splenic fever only. If, on the other hand, the operation is performed after a greater number of hours—say, between twelve and twenty, according to the season of the year—then the inoculation of the blood will communicate, at one and the same time, splenic fever and septicÆmia—acute septicÆmia, as it may be called, because of the rapid inflammatory disorders that the septic vibrio causes in the inoculated animal. The two diseases may be developed simultaneously in the inoculated animal, but generally one precedes the other. The septic contagium is the quickest in its action; it generally causes death before the splenic fever has had time to develop itself and to produce appreciable effects.

We are now in a position to explain all the contradictory results obtained by MM. Jaillard and Leplat on one side, and by Davaine on the other. In a country which splenic fever had made famous, the DÉpartement d'Eure-et-Loir, they had asked for a little splenic fever blood. Now, what takes place in a farm where an animal has died of this disease? The dead body is thrown upon a dungheap, or into some shed or stall, until the knacker's cart happens to pass. The knacker takes his own time, and the body often remains there twenty-four or forty-eight hours. The blood taken from this animal is more or less invaded by putrefaction, and vibrios are mingled with the bacteria of splenic fever, the development of which is arrested the moment the animal dies. In short, it may be easily conceived that an experimenter writing to Chartres to procure some splenic fever blood might, without his knowledge, or the knowledge of his correspondent, receive blood at the same time both splenic and septic. And this septicÆmia is sometimes manifold, for a special septicÆmia may be said to correspond to every sort of vibrio of putrefaction.

Such were the circumstances which, without their being aware of it, accompanied Jaillard and Leplat's researches upon splenic fever infection. This impression will be derived from reading the successive notes laid by them before the Academy of Sciences. The blood of the cow which had died of splenic fever, sent from the knacker's establishment of Sours, and the blood of the sheep sent by M. Boutet, must both have been taken from the bodies of animals which had been dead a sufficient number of hours to render their blood both splenic and septic; and it was septicÆmia, so prompt in its action, that had killed the rabbits of Jaillard and Leplat. As the examination of the blood of these animals showed no signs of bacteria, they had concluded, with great apparent truth, that the inoculation of splenic blood could cause death without any appearance of these organisms, even while the blood used for inoculation was full of them. The presence of septic vibrios in the blood of the inoculated rabbits escaped their notice. When Davaine replied that Jaillard and Leplat had not worked with pure splenic blood he had hit upon the truth, but he could not give plausible reasons for it. The contest was carried on by experiments in which, on both sides, truth and error were closely blended.

The work of M. Paul Bert, at the close of 1876, was surrounded with circumstances no less complex. To thoroughly understand them we must call to mind Pasteur's discovery as to the mode of reproducing the anaerobic germs of putrefaction. These vibrios reproduce themselves by spores. In the vibrio of acute septicÆmia this is the mode of generation. Short or long jointed filaments show themselves studded with brilliant points, which are precisely the spores of which we speak. Experience proves that these spores resist perfectly the poisonous action of compressed oxygen. Inoculating an animal with blood which is at the same time septic and splenic, after the blood has been compressed, the septic germs, remaining alive, produce death, although neither bacteria nor filaments may be perceptible in its blood at the moment of death. It was likewise from Chartres that M. Paul Bert obtained his supply of splenic fever blood. The blood he had received was without doubt not only splenic but also septic. The filaments of bacteria and the filaments of septic vibrios had perished under the influence of the compressed oxygen; but the spores were there, and the great pressure of oxygen gas had not affected them. The new contagium which had appeared, and which had killed the inoculated animals, was due to these spores.

As regards the proof that this virulence in the blood of the body of an animal which has died of splenic fever is really the effect of the septic vibrio, Pasteur, assisted by Joubert and a new assistant, M. Chamberland, has given that proof, as he did in the case of the bacterium of splenic fever, by resorting to the method of successive cultivations in an artificial medium. These cultivations, however, of the septic vibrio require very special precautions and conditions. They should be carried on in as perfect a vacuum as it is possible to obtain, or in contact with carbonic acid gas without the presence of air. In contact with air the cultivations of septic vibrios would prove sterile, because the vibrio is exclusively anaerobic and air kills it. If a spore of this organism could germinate in contact with the air, the product of the germination would be at once arrested and would perish by the action of the oxygen. It is exactly the contrary with the bacilli of splenic fever, which prove sterile in a vacuum or in presence of carbonic acid gas. If one of the spores of the splenic fever bacillus (for it also produces spores) could germinate, the product of the germination, deprived of free oxygen, would at once perish. And, to mention in passing a very ingenious experiment of Pasteur's, we thus obtain a means of separating by culture the bacillus of splenic fever from the septic vibrio when they are temporarily associated together. If this mixture of pathogenic organisms is cultivated in contact with the air, the bacilli of splenic fever alone will be developed. If this same mixture is cultivated without air, either in a vacuum or in carbonic acid gas, the septic vibrio alone will be developed. This device of culture is one of the best which can be employed to demonstrate that the blood of a body dead from splenic fever possesses immediately after death a single contagium, that of splenic fever, and that twenty-four hours after death, on the contrary, there are two contagia, that of splenic fever and that of septicÆmia.

Some months ago a very hot discussion arose between Pasteur and a commission formed principally of professors of the veterinary school in Turin, regarding the facts above mentioned. One experiment, in the success of which Pasteur was extremely interested, had been made at this school. Instead of employing pure splenic fever blood, free from all contagium, the Italian professors, whether from ignorance of the preceding facts or from inadvertence, employed the blood of a diseased sheep, which, from their own showing, had been dead more than twenty-four hours. Pasteur immediately wrote, pointing out that the commission had done wrong in using blood which must have been at the same time splenic and septic. The Turin professors grew angry, and affirmed that this assertion of Pasteur's was incorrect; that this sheep's blood had been studied with care, and that no filaments had been found in it except those of splenic fever; and it would, moreover, be marvellous, they added ironically, that Pasteur from the depths of his laboratory in Paris should be able to assert that this blood was mixed with septic poison, whilst they, good observers, armed with a microscope, had had this sheep's blood under their eyes. Pasteur contented himself with replying that his assertion rested upon a principle, and that he was perfectly able, without having seen the blood of the sheep, to affirm that under the conditions in which it had been collected that blood was septic. A public correspondence ensued, but no understanding could be come to. Pasteur then offered to go himself to Turin, in order to demonstrate upon as many bodies of sheep dead of splenic fever as they would like to give him, that the blood of these dead bodies—at the end of twenty-four hours if in the month of March, and in twelve or fifteen hours if in the month of June, would be found to be both splenic and septic. Pasteur also proposed, by appropriate cultures, to withdraw at pleasure the splenic fever poison or the septic poison, or the two together, at the choice of the Italians. The Italians, however, shrank from Pasteur's proposal to pay them a visit in order to convince them of their error.

The clearness and certainty of Pasteur's assertions are celebrated, but what gives such authority to all that he advances is, as M. Paul Bert once said, that Pasteur's boldness of assertion is only equalled by his diffidence when he has not experiment to back him up. He never fights except on ground with which he has made himself familiar, but then he fights with such resolution, and sometimes with such impetuosity, that one might say to his adversary, whoever he be, 'Je vous plains de tomber dans ses mains redoutables.'

'Take care!' said a member of the Academy of Sciences to a member of the Academy of Medicine, who a short time after the incident just related was proposing scientifically to 'strangle' Pasteur, 'take care! Pasteur is never mistaken.'

One day, in 1879, a professor attached to a faculty of medicine in one of the provinces announced to the Academy of Sciences that he had found, in the blood of a woman who had died in a hospital after two weeks' illness from severe puerperal fever, a considerable number of motionless filaments, simple or jointed, transparent, straight, or bent, which belonged to the genus Leptothrix. Engaged in studies on puerperal fever, and having never met with a fact of this kind in his researches, Pasteur wrote at once to this professor to ask him for a specimen of the infected blood. The blood arrived at the laboratory, and some days after Pasteur wrote to the doctor, 'Your leptothrix is nothing else than the bacterium of splenic fever.'

This answer perplexed the doctor very much. He wrote to Pasteur that he did not dispute the affirmation, but that he proposed to control it; that if he found he had been in error he would publish it.

Pasteur offered to send him guinea-pigs which had been inoculated with splenic fever. 'You will receive them still living; they will die under your eyes. You will make the autopsy and you will yourself recognise your leptothrix.' The doctor accepted the test. Pasteur inoculated three guinea-pigs, had them placed in a cage and sent by rail to the professor. They arrived the following morning and died twenty-four hours afterwards under the doctor's own eyes. The first had been inoculated with the infectious blood of the dead woman, the second with the bacterium of splenic fever blood from Chartres, the third with the blood of a cow which had died of splenic fever in the Jura. At the autopsy it was impossible to discover the slightest difference in the blood of the three animals. Not only the blood but the internal organs, and especially the spleen, were in exactly the same condition.

Then, in the most honourable manner, the doctor hastened to state, in a communication to the Academy of Sciences, that he regretted doubly not having known about splenic fever the year before, as he might have been able, on the one hand, to diagnose the formidable complication which had manifested itself in the woman who died on April 4, 1878, and, on the other hand, to have traced out the mode of contamination which now eluded him. He had, however, succeeded in learning a few details regarding the unhappy woman. She was a charwoman, and lived in a little room adjoining the stables of a horse-dealer. Through these stables a large number of horses passed continually.

But to return to our septic vibrios. If air destroys them, if their culture is impossible in contact with air, how can septicÆmia exist, since air is everywhere present? How can blood exposed to the air become septic from particles of dust on the surface of objects or which the air holds in suspension? Where can the septic germs be formed? The objection seems a serious one, but it disappears before a very simple experiment. Take some serum from the abdomen of a guinea-pig which has died of acute septicÆmia. It will be found full of septic vibrios in process of generation by fission. Let this liquid be then exposed to the contact of air, with the precaution of giving a certain depth to the liquid—say, a centimeter of depth. In some hours, if examined with the microscope, the following curious spectacle will be witnessed: In the upper layers the oxygen of the air is absorbed, which is manifested by the already changed colour of the liquid. There the filamentous vibrio dies, and disappears under the form of fine amorphous granulations deprived of virulence. At the bottom of this layer of one centimeter in thickness, on the contrary, the vibrios, protected from the approach of oxygen by those of their own kind which have perished above them, continue to multiply by fission until by degrees they pass into the state of spores; so that instead of moving threads of all dimensions, the length of which sometimes even extends beyond the field of the microscope, nothing is now seen but a dust of brilliant isolated specks, upon which the oxygen of the air has no action. It is thus that a dust of septic germs can be formed even in contact with air. And thus it becomes possible to understand how anaerobic organisms may be sown in putrescible liquids by the dust suspended in the atmosphere. Thus also may be explained the permanence of putrid diseases, even of those which are caused by anaerobic microbes, that cannot live in the atmosphere and which escape destruction by becoming spores.

By means of these experiments, as unexpected as they were conclusive, Pasteur had demonstrated that Jaillard and Leplat had not really inoculated their rabbits with an amorphous virus, liquid or solid, but with a virus constituted of a living microscopic organism—in other words, with a true ferment. By the side of the parasite of splenic fever we have thus a fresh example of a living animated virus, with germs forming dust. And the extraordinary thing is that among the microbes of special maladies—which they produce by penetrating and multiplying in the bodies of animals—are to be found aerobies like the bacilli of splenic fever, and anaerobies like the vibrios of acute septicÆmia.

II.

In these two virulent maladies, then, splenic fever and septicÆmia, the researches of Pasteur had clearly established the parasitic theory. A grand and novel opening was made for future studies on the origin of diseases. Yet, judging from the surprising differences which separate septicÆmia and splenic fever, we can foresee that should the future, copying the past, in regard to this and still more recent discoveries, have in store, as it no doubt has, the knowledge of new microbes of disease, the specific properties of these microscopic organisms will demand, for each new exploration, ceaselessly repeated efforts, not only to make the existence of these organisms evident, but also to furnish decisive proofs of their morbific power. But the question which may be considered as already solved is the non-spontaneity of these infectious microbes. By what is called spontaneous disease is meant parasitic disease. But in the present state of science spontaneous disease has no more existence than spontaneous generation. Such aphorisms, however, are not allowed to pass without occasional contradictions, all the more vehement from their rarity. At the International Medical Congress held in London, August 1881, Dr. Bastian, who practises in one of the principal hospitals of London, declared that though he was unable to deny the existence of parasitic diseases, yet, in his opinion, the microbes were the effect and not the cause of these diseases.

'Is it possible,' cried Pasteur, who was present at the meeting, 'that at this day such a scientific heresy should be held? My answer to Dr. Bastian will be short. Take the limb of an animal and crush it in a mortar; let there be diffused in this limb, around these crushed bones, as much blood, or any other normal or abnormal liquid as you please. Take care only that the skin of the limb is neither torn nor laid open, and I defy you to exhibit on the following day, or during all the time the malady lasts, the least microscopic organism in the humours of this limb.'

After the example of Liebig in 1870, Dr. Bastian did not accept the challenge.

But if a disease like splenic fever is carried by a microbe, this microbe is under the influence of the medium in which it finds itself. It does not develop everywhere. Easily inoculable and fatal to the ox, the sheep, the rabbit, and the guinea pig, splenic fever is very rare in the dog and in the pig. These must be inoculated several times before they contract the disease, and even then it is not always possible to produce it. Again, there are some creatures which are never assailable by it. It can never be taken by fowls. In vain they are inoculated with a considerable quantity of splenic blood; it has no effect upon them. This invulnerability had very much struck Pasteur and his two assistants, Joubert and Chamberland. What was it in the body of a fowl that enabled it to thus resist inoculations of which the most infinitesimal quantity sufficed to kill an ox? They proved by a series of experiments that the microbe of splenic fever does not develop when subjected to a temperature of 44° Centigrade. Now, the temperature of birds being between 41 and 42 degrees, may it not be, said Pasteur, that the fowls are protected from the disease because their blood is too warm—not far removed from the temperature at which the splenic fever organism can no longer be cultivated? Might not the vital resistance encountered in the living fowl suffice to bridge over the small gap between 41-42, and 44-45 degrees? For we must always allow for a certain resistance in all living creatures to disease and death. No doubt, life to a parasite in the body of an animal would not be as easy as in a cultivating liquid contained in a glass vessel. If the inoculating microbe is aerobic, it can only be cultivated in blood by taking away the oxygen from the globules, which retain it with a certain force for their own life. Nothing was more legitimate than to suppose that the globules of the blood of the fowl had such an avidity for oxygen that the filaments of the splenic parasite were deprived of it, and that their multiplication was thus rendered impossible. This idea conducted Pasteur and his assistants to new researches. 'If the blood of a fowl was cooled,' they asked, 'could not the splenic fever parasite live in this blood?'

The experiment was made. A hen was taken, and, after inoculating it with splenic fever blood, it was placed with its feet in water at 25 degrees. The temperature of the blood of the hen went down to 37 or 38 degrees. At the end of twenty-four hours the hen was dead, and all its blood was filled with splenic fever bacteria.

But if it was possible to render a fowl assailable by splenic fever simply by lowering its temperature, is it not also possible to restore to health a fowl so inoculated by warming it up again? A hen was inoculated, subjected, like the first, to the cold-water treatment, and when it became evident that the fever was at its height it was taken out of the water, wrapped carefully in cotton wool, and placed in an oven at a temperature of 35 degrees. Little by little its strength returned; it shook itself, settled itself again, and in a few hours was fully restored to health. The microbe had disappeared. Hens killed after having been thus saved, no longer showed the slightest trace of splenic organisms.

How great is the light which these facts throw upon the phenomenon of life in its relation to external physical conditions, and what important inferences do they warrant as to the influence of external media and conditions upon the life and development of living contagia! There have been great discussions in Germany and France upon a mode of treatment in typhoid fever, which consists in cooling the body of the patient by frequently repeated baths. The possible good effects of this treatment may be understood when viewed in conjunction with the foregoing experiment on fowls. In typhoid fever the cold arrests the fermentation, which may be regarded as at once the expression and the cause of the disease, just as, by an inverse process, the heat of the body arrests the development of the splenic fever microbe in the hen.