But his own conviction had not yet penetrated the minds of his adversaries, and M. Trécul was still supporting his hypothesis of transformations, the so-called proofs of which, according to Pasteur, rested on a basis of confused facts tainted with involuntary errors due to imperfect experiments. In December, 1873, at a sitting of the Academy, he presented M. Trécul with a few little flagons, in which he had sown some pure seed of _penicillium glaucum_, begging him to accept them and to observe them at his leisure, assuring him that it would be impossible to find a trace of any transformation of the spores into yeast cells. “When M. Trécul has finished the little task which I am soliciting of his devotion to the knowledge of truth,” continued Pasteur, “I shall give him the elements of a similar work on the _mycoderma vini_; in other words, I shall bring to M. Trécul some absolutely pure _mycoderma vini_ with which he can reproduce his former experiments and recognize the exactness of the facts which I have lately announced.” Pasteur concluded thus: “The Academy will allow me to make one last remark. It must be owned that my contradictors have been peculiarly unlucky in taking the occasion of my paper on the diseases of beer to renew this discussion. How is it they did not understand that my process for the fabrication of inalterable beer could not exist if beer wort in contact with air could present all the transformations of which they speak? And that work on beer, entirely founded as it is on the discovery and knowledge of some microscopic beings, has it not followed my studies on vinegar, on the mycoderma aceti and on the new process of acetification which I have invented? Has not that work been followed by my studies on the causes of wine diseases and the means of preventing them, still founded on the discovery and knowledge of non-spontaneous microscopic beings? Have not these last researches been followed by the discovery of means to prevent the silkworm disease, equally deducted from the study of non-spontaneous microscopic beings? “Are not all the researches I have pursued for seventeen years, at the cost of many efforts, the product of the same ideas, the same principles, pushed by incessant toil into consequences ever new? The best proof that an observer is in the right track lies in the uninterrupted fruitfulness of his work.”