mentation must have been in Pasteur's mind; and then came the suggestion, "What would be most desirable is to push those studies far enough to prepare the road for a serious research into the origin of various diseases." If the changes in lactic, alcoholic and butyric fermentations are due to minute living organisms, why should not the same tiny creatures make the changes which occur in the body in the putrid and suppurative diseases? With an accurate training as a chemist, having been diverted in his studies upon fermentation into the realm of biology, and nourishing a strong conviction of the identity between putrefactive changes of the body and fermentation, Pasteur was well prepared to undertake investigations which had hitherto been confined to physicians alone.

So impressed was he with the analogy between fermentation and the infectious diseases that, in 1863, he assured the French Emperor of his ambition "to arrive at the knowledge of the causes of putrid and contagious diseases." After a study upon the diseases of wines, which has had most important practical bearings, an opportunity arose which changed the whole course of his career, and profoundly influenced the development of medical science. A disease of the silkworm had, for some years, ruined one of the most important industries in France, and in 1865 the Government asked Pasteur to give up his laboratory work and teaching, and to devote his whole energies to the task of investigating it. The story of the brilliant success which followed years of application to the problem will be read with deep interest by every student of science. It was the first of his victories in the application of the experimental methods of a trained chemist to the problems of biology, and it placed his name high in the group of the most illustrious benefactors of practical industries.

In a series of studies on the diseases of beer, and on the mode of production of vinegar, he became more and more convinced that these studies on fermentation had given him the key to the nature of the infectious diseases. It is a remarkable fact that the distinguished English philosopher of the seventeenth century, the man who more than anyone else of his century appreciated the importance of the experimental method, Robert Boyle, had said that he who could discover the nature of ferments and fermentation, would be more capable than anyone else of explaining the nature of certain diseases. In 1876 there appeared in Cohn's "Beiträge zur Morphologie der

Pflanzen" (II, 277-310), a paper on the "Etiology of Anthrax" by a German district physician in Wollstein, Robert Koch (Fig. 105), which is memorable in our literature as the starting point of a new method of research into the causation of infectious diseases. Koch demonstrated the constant presence of germs in the blood of animals

dying from the disease. Years before, those organisms had been seen by Pollender and Davaine, but the epoch-making advance of Koch was to grow those organisms in a pure culture outside the body, and to produce the disease artificially by inoculating animals with the cultures. Koch is really our medical Galileo, who, by means of a new technique, pure cultures and isolated staining, introduced us to a

new world. In 1878, followed his study on the "Etiology of Wound Infections," in which he was able to demonstrate conclusively the association of micro-organisms with the disease. (Fig. 106.) Upon those two memorable researches made by a country doctor rests the modern science of bacteriology.

The next great advance was the discovery by Pasteur of the possibility of so attenuating, or weakening, the poison that an animal inoculated had a slight attack, recovered and was then protected against the disease. More than eighty years had passed since on May 14, 1796, Jenner had vaccinated a child with cowpox and proved that a slight attack of one disease protected the body from a disease of an allied nature. An occasion equally famous in the history of medicine was a day in 1881, when Pasteur determined that a flock of sheep vaccinated with the attenuated virus of anthrax remained well, when every one of the unvaccinated infected from the same material had died. Meanwhile, from Pasteur's researches on fermentation and spontaneous generation, a transformation had been initiated in the practice of surgery, which, it is not too much to say, has proved one of the greatest boons ever conferred upon humanity. It had long been recognized that, now and again, a wound healed without the formation of pus, that is, without suppuration, but both spontaneous and operative wounds were almost invariably associated with that process; and, moreover, they frequently became putrid, as it was then called, infected, as we should say, the general system became involved and the patient died of blood poisoning. So common was this, particularly in old, ill-equipped hospitals, that many surgeons feared to operate, and the general mortality in all surgical cases was very high. Believing that it was from outside that the germs came which caused the decomposition of wounds, just as from the atmosphere the sugar solution got the germs which caused the fermentation, a young surgeon in Glasgow, Joseph Lister, applied the principles of Pasteur's experiments to their treatment. From Lister's original paper* I quote the following: "Turning now to the question how the atmosphere produces decomposition of organic substances, we find that a flood of light has been thrown upon this most important subject by the philosophic researches of M. Pasteur, who has demonstrated by thoroughly convincing evidence that it is not to its oxygen or to any of its gaseous

* Lancet, March 16, 1867. [Cf. Camac: Epoch-making Contributions, etc., 1909, p. 7.-Ed.]