Pasteur, then in full possession of all the qualities of his genius, was feeling the sort of fever known to great scientists, great artists, great writers: the ardent desire of finding, of discovering something he could leave to posterity. Interrupted by these belated contradictors when he wanted to be going forward, he only restrained his impatience with difficulty.
His old master, Balard, appealed to him in the Académie itself (January 22, 1872), in the name of their old friendship, to disregard the attacks of his adversaries, instead of wasting his time and his strength in trying to convince them. He reminded him of all he had achieved, of the benefits he had brought to the industries of wine, beer, vinegar, silkworms, etc., and alluded to the possibility foreseen by Pasteur himself of preserving mankind from some of the mysterious diseases which were perhaps due to germs in atmospheric air. He ended by urging him to continue his studies peacefully in the laboratory built for him, and to continue the scientific education of young pupils who might one day become worthy successors of Van Tieghem, Duclaux, Gernez, Raulin, etc.... thus forming a whole generation of young scientists instructed in Pasteur’s school.
M. Duclaux wrote to him in the same sense: “I see very well what you may lose in that fruitless struggle—your rest, your time and your health; I try in vain to see any possible advantage.”
But nothing stopped him; neither Balard’s public advice, his pupils’ letters, even J. B. Dumas’ imploring looks. He could not keep himself from replying. Sometimes he regretted his somewhat sharp language, though—in his own words—he never associated it with feelings of hostility towards his contradictors as long as he believed in their good faith; what he wanted was that truth should have the last word. “What you lack, M. Frémy, is familiarity with a microscope, and you, M. Trécul, are not accustomed to laboratories!” “M. Frémy is always trying to displace the question,” said Pasteur, ten months after M. Balard’s appeal.
Whilst M. Frémy disputed, discussed, and filled the Académie with his objections, M. Trécul, whose life was somewhat misanthropical and whose usually sad and distrustful face was seen nowhere but at the Institute, insisted slowly, in a mournful voice, on certain transformations of divers cells or spores from one into the other. Pasteur declared that those ideas of transformation were erroneous; but—and there lay the interest of the debate—there was one of those transformations that Pasteur himself had once believed possible: that of the mycoderma vini, or wine flower, into an alcoholic ferment under certain conditions of existence.
A modification in the life of the mycoderma when submerged had led him to believe in a transformation of the mycoderma cells into yeast cells. It was on this question, which had been left in suspense, that the debate with Trécul came to an end, leaving to the witnesses of it a most vivid memory of Pasteur’s personality—inflexible when he held his proofs, full of scruples and reserve when seeking those proofs, and accepting no personal praise if scientific truth was not recognized and honoured before everything else.
On November 11 Pasteur said: “Four months ago doubts suddenly appeared in my mind as to the truth of the fact in question, and which M. Trécul still looks upon as indisputable.... In order to disperse those doubts I have instituted the most numerous and varied experiments and I have not succeeded through those four months in satisfying myself by irrefragable proofs; I still have my doubts. Let this example show to M. Trécul how difficult it is to conclude definitely in such delicate studies.”
Pasteur studied the scientific point for a long time, for he never abandoned a subject, but was ever ready to begin again after a failure. He modified the disposition of his first tests, and by the use of special vessels and slightly complicated apparatus succeeded in eliminating the only imaginable cause of error—the possible fall, during the manipulations, of exterior germs, that is, the fortuitous sowing of yeast cells. After that he saw no more yeast and no more active alcoholic fermentation; he had therefore formerly been the dupe of a delusion. In his Studies on Beer Pasteur tells of his error and its rectification: “At a time when ideas on the transformations of species are so readily adopted, perhaps because they dispense with rigorous experimentation, it is somewhat interesting to consider that in the course of my researches on microscopic plants in a state of purity I once had occasion to believe in the transformation of one organism into another, the transformation of the mycoderma vini or cerevisiae into yeast, and that this time I was in error; I had not avoided the cause of illusion which my confirmed confidence in the theory of germs had so often led me to discover in the observations of others.”
“The notion of species,” writes M. Duclaux, who was narrowly associated with those experiments, “was saved for the present from the attacks directed against it, and it has not been seriously contested since, at least not on that ground.”
Some failures are blessings in disguise. When discovering his mistake, Pasteur directed his attention to a strange phenomenon. We find in his book on beer—a sort of laboratory diary—the following details on his observation of the growth of some mycoderma seed which he had just scattered over some sweetened wine or beer-wort in small china saucers.