“The most decisive of all the proofs which can be invoked in favour of the possible correlation between a determined affection and the presence of a micro-organism, is that afforded by the method of cultures of organisms in a state of purity; a method by which I have solved, within the last twenty-two years, the chief difficulties relative to fermentations properly so called; notably the important question, much debated formerly, of the correlation which exists between those fermentations and their particular ferments.” He then pointed out that if, after gathering either blood or pus immediately before or immediately after the death of a plague patient, one could succeed in discovering the micro-organism, and then in finding for that microbe an appropriate culture medium, it would be advisable to inoculate with it animals of various kinds, perhaps monkeys for preference, and to look for the lesions capable of establishing relations from cause to effect between that organism and the disease in mankind. He did not hide from himself the great difficulties to be met with in experimenting; for, after discovering and isolating the organism, there is nothing to indicate _a priori_ to the experimentalist an appropriate culture medium. Liquids which suit some microbes admirably are absolutely unsuitable to others. Take, for instance, the microbe of chicken-cholera, which will not develop in beer yeast; a hasty experimentalist might conclude that the chicken-cholera is not produced by a micro-organism, and that it is a spontaneous disease with unknown immediate causes. “The fallacy would be a fatal one,” said Pasteur, “for in another medium, say, for instance, in chicken-broth, there would be a virulent culture.” In these researches on the plague, then, various mediums should be tried; also the character, either aërobic or anaërobic, of the microbe should be present to the mind. “The sterility of a culture liquid may come from the presence of air and not from its own constitution; the septic vibrio, for instance, is killed by oxygen in air. From this last circumstance it is plain that culture must be made not only in the presence of air but also in a vacuum or in the presence of pure carbonic acid gas. In the latter case, immediately after sowing the blood or humour to be tested, a vacuum must be made in the tubes, they must be sealed by means of a lamp, and left in a suitable temperature, usually between 30° C. and 40° C.” Thus he prepared landmarks for the guidance of scientific research on the etiology of the plague. * * * * * Desiring as Pasteur did that the public in general should take an interest in laboratory research, he sent to his friend Nisard the number of the _Bulletin of the Académie de Médecine_ which contained a first communication on chicken-cholera, and also his paper on the plague.