But there is still another class of chemical phenomena where the life without air of microscopic organisms is fully shown. Pasteur proved that in the special fermentation which bears the name of putrefaction the _primum movens_ of the putrefaction resides in microscopic vibrios of absolutely the same order as those which compose the butyric ferment. The fermentation of sugar, of mannite, of gums, of lactate of lime, by the butyric vibrio, so closely resembles the phenomena of putrefaction, that one might call these fermentations the putrefaction of sugar and of the other products. If it has been thought right to call the fermentation of animal matters putrefaction, it is because at the moment of the decomposition of fibrine, of albumen, of blood, of gelatine, of the substance of the tendons, &c., the sulphur, and even the phosphorus, which enter into their composition give rise to putrid odours, due to the evil-smelling gases of sulphur and phosphorus. The phenomena of putrefaction being then simply fermentations, differing only in regard to the chemical composition of the fermenting matters, Liebig naturally included them in his general theory of the decomposition of organic matters after death. At a period long antecedent to Pasteur's labours it had been established that there existed in putrefying matters fungi or microscopic animalculæ, and the idea had taken shape that these creatures might have an influence in the phenomena. The proofs were wanting, but the notion of a possible relation remained. We may read in his 'Lessons on Chemistry' with what disdain Liebig mentioned these hypothetical opinions. 'Those who pretend to explain the putrefaction of animal substances by the presence of animalculæ,' he wrote, 'reason very much like a child who would explain the rapidity of the Rhine by attributing it to the violent motions imparted to it in the direction of Bingen by the numerous wheels of the mills of Mayence. Is it possible to consider plants and animals as the causes of the destruction of other organisms when their own elements are condemned to undergo the same decompositions as the creatures which have preceded them? If the fungus is the cause of the destruction of the oak, if the microscopic animalcula is the cause of the putrefaction of the dead elephant, I would ask in my turn what is the cause which determines the putrefaction of the fungus or of the microscopic animalcula when life is withdrawn from these two organisms?' Thirty-two years later, and after Pasteur had accumulated, during more than twenty years, proof upon proof that the theory of Liebig would not stand examination, a physician of Paris, M. Bouillaud, asked, with the insistent voice of a querulous octogenarian: 'Let M. Pasteur then tell us here, in presence of the Académie de Médecine, what are the ferments of the ferments.'