“It will be seen,” wrote M. Duclaux, “to what heights he had raised the debate; by changing the mode of interpretation of known facts he brought out a new theory.” But this new theory raised a chorus of controversy. Pasteur held to his proofs; he recalled what he had published concerning the typical ferment, the yeast of beer, an article inserted in the reports of the Académie des Sciences for 1861, and entitled, _The Influence of Oxygen on the Development of Yeast and on Alcoholic Fermentation_. In this article Pasteur, à propos of the chemical action connected with vegetable life, explained in the most interesting manner the two modes of life of the yeast of beer. 1. The yeast, placed in some sweet liquid in contact with air, assimilates oxygen gas and develops abundantly; under those conditions, it practically works for itself only, the production of alcohol is insignificant, and the proportion between the weight of sugar absorbed and that of the yeast is infinitesimal. 2. But, in its second mode of life, if yeast is made to act upon sugar without the action of atmospheric air, it can no longer freely assimilate oxygen gas, and is reduced to abstracting oxygen from the fermentescible matter. “It seems therefore natural,” wrote Pasteur, “to admit that when yeast is a ferment, acting out of the reach of atmospheric air, it takes oxygen from sugar, that being the origin of its fermentative character.” It is possible to put the fermentative power of yeast through divers degrees of intensity by introducing free oxygen in variable quantities. After comparing the yeast of beer to an ordinary plant, Pasteur added that “the analogy would be complete if ordinary plants had an affinity for oxygen so strong as to breathe, by withdrawing that element from unstable components, in which case they would act as ferments on those substances.” He suggested that it might be possible to meet with conditions which would allow certain inferior plants to live away from atmospheric air in the presence of sugar, and to provoke fermentation of that substance after the manner of beer yeast. He was already at that time scattering germs of ideas, with the intention of taking them up later on and experimenting on them, or, if time should fail him, willingly offering them to any attentive scientist. These studies on beer had brought him back to his former studies, to his great delight. “What a sacrifice I made for you,” he could not help saying to Dumas, with a mixture of affection and deference, and some modesty, for he apparently forgot the immense service rendered to sericiculture, “when I gave up my studies on ferments for five whole years in order to study silkworms!!!”