Before replying to this argument, which Liebig and M. Bouillaud believed to be irrefutable, Pasteur, wishing to mark all the phases of the phenomena, expounded in a short preamble the part played by atmospheric oxygen in the destruction of animal and vegetable matters after death. It is easy to understand, indeed, that fermentation and putrefaction only represent the first phase of the return to the atmosphere and to the soil of all that has lived. Fermentations and putrefactions give rise to substances which are still very complex, although they represent the products of decomposition of fermentable matters. When sugar ferments, a large proportion of it becomes gas; but alongside of the carbonic acid gas which is formed, and which is, indeed, a partial return of the sugar to the atmosphere, new substances, such as alcohol, succinic acid, glycerine, and materials of yeast, are produced. When the flesh of animals putrifies, certain products of decomposition, also very complex, are formed with the vapour of water and the other gases of putrefaction. Where, then, does nature find the agents of destruction of these secondary products? The great fact of the destruction of animal and vegetable matters is accomplished by slow combustion, through the appropriation of atmospheric oxygen. Here, again, one must banish from science the preconceived views which assumed that the oxygen seized directly on the organic matter after death, and that this matter was consumed by purely chemical processes. It is life that presides over this work of death. If fermentation and putrefaction are principally the work of microscopic anaérobies, living without free oxygen, the slow combustion is found very largely, if not exclusively, to depend upon a class of infinitely small aérobies. It is these last which have the property of consuming the oxygen of the air. It is these lower organisms which are the powerful agents in the return to the atmosphere of all which has lived. Mildew, mould, bacteria, which we have already noticed, monads, two thousand of which would go to make up a millimeter, all these microscopic organisms are charged with the great work of re-establishing the equilibrium of life by giving back to it all that it has formed.