While Pasteur was going from the Observatoire cellars to the Mer de Glace, Pouchet was gathering air on the plains of Sicily, making experiments on Etna, and on the sea. He saw everywhere, he wrote, “air equally favourable to organic genesis, whether surcharged with detritus in the midst of our populous cities, or taken on the summit of a mountain, or on the sea, where it offers extreme purity. With a cubic decimetre of air, taken where you like, I affirm that you can ever produce legions of microzoa.” And the heterogenists proclaimed in unison that “everywhere, strictly everywhere, air is constantly favourable to life.” Those who followed the debate nearly all leaned towards Pouchet. “I am afraid,” wrote a scientific journalist in _La Presse_ (1860), “that the experiments you quote, M. Pasteur, will turn against you.... The world into which you wish to take us is really too fantastic....” And yet some adversaries should have been struck by the efforts of a mind which, while marching forward to establish new facts, was ever seeking arguments against itself, and turned back to strengthen points which seemed yet weak. In November, Pasteur returned to his studies on fermentations in general and lactic fermentation in particular. Endeavouring to bring into evidence the animated nature of the lactic ferment, and to indicate the most suitable surroundings for the self-development of that ferment, he had come across some complications which hampered the purity and the progress of that culture. Then he had perceived another fermentation, following upon lactic fermentation and known as butyric fermentation. As he did not immediately perceive the origin of this butyric acid--which causes the bad smell in rancid butter--he ended by being struck by the inevitable coincidence between the (then called) infusory animalculæ and the production of this acid.