He tried to make them understand scientifically the well known fact of the transformation of wine into vinegar. He showed that all the work came from a little plant, a microscopic fungus, the mycoderma aceti. After exhibiting an enlarged picture of that mycoderma, Pasteur explained that the least trace of that little vinegar-making plant, sown on the surface of any alcoholic and slightly acid liquid, was sufficient to produce a prodigious extension of it; in summer or artificial heat, said Pasteur, a surface of liquid of the same area as the Orleans Lecture room could be covered in forty-eight hours. The mycodermic veil is sometimes smooth and hardly visible, sometimes wrinkled and a little greasy to the touch. The fatty matter which accompanies the development of the plant keeps it on the surface, air being necessary to the plant; it would otherwise perish and the acetification would be arrested. Thus floating, the mycoderma absorbs oxygen from the air and fixes it on the alcohol, which becomes transformed into acetic acid.
Pasteur explained all the details in his clear powerful voice. Why, in an open bottle, does wine left to itself become vinegar? Because, thanks to the air, and to the mycoderma aceti (which need never be sown, being ever mixed with the invisible dusts in the air), the chemical transformation of wine into vinegar can take place. Why does not a full, closed bottle become acetified? Because the mycoderma cannot multiply in the absence of air. Wine and air heated in the same vessel will not become sour, the high temperature having killed the germs of mycoderma aceti both in the wine itself and in the dusts suspended in the air. But, if a vessel containing wine previously heated is exposed to the free contact of ordinary air, the wine may become sour, for, though the germs in the wine have been killed, other germs may fall into it from the air and develop.
Finally, if pure alcoholized water does not become acetified, though germs can drop into it from the air, it is because it does not offer to those germs the food necessary to the plant—food which is present in wine but not in alcoholized water. But if a suitable aliment for the little plant is added to the water, acetification takes place.
When the acetification is complete, the mycoderma, if not submerged, continues to act, and, when not arrested in time, its oxidating power becomes dangerous; having no more alcohol to act upon, it ends by transforming acetic acid itself into water and carbonic acid gas, and the work of death and destruction is thus achieved.
Speaking of that last phase of the mycoderma aceti, he went on to general laws—laws of the universe by which all that has lived must disappear. “It is an absolute necessity that the matter of which living beings are formed should return after their death to the ground and to the atmosphere in the shape of mineral or gaseous substances, such as steam, carbonic acid gas, ammoniac gas or nitrogen—simple principles easily displaced by movements of the atmosphere and in which life is again enabled to seek the elements of its indefinite perpetuity. It is chiefly through acts of fermentation and slow combustion that this law of dissolution and return to a gaseous state is accomplished.”
Coming back to his special subject, he pointed out to vinegar manufacturers the cause of certain failures and the danger of certain errors.
It was imagined for instance that some microscopic beings, anguillulæ, of which Pasteur projected an enlarged wriggling image on the screen, and which were to be found in the tubs of some Orleans vinegar works, were of some practical utility. Pasteur explained their injurious character: as they require air to live, and as the mycoderma, in order to accomplish its work, is equally dependent on oxygen, a struggle takes place between the anguillulæ and the mycoderma. If acetification is successful, if the mycoderma spreads and invades everything, the vanquished anguillulæ are obliged to take refuge against the sides of the barrel, from which their little living army watches the least accidental break of the veil. Pasteur, armed with a magnifying glass, had many times witnessed the struggle for life which takes place between the little fungi and the tiny animals, each fighting for the surface of the liquid. Sometimes, gathering themselves into masses, the anguillulæ succeed in sinking a fragment of the mycodermic veil and victoriously destroying the action of the drowned plants.
Pasteur related all this in a vivid manner, evidently happy that his long and delicate laboratory researches should now pass into the domain of industry. He had been pleased to find that some Orleans wine merchants heated wine according to his advice in order to preserve it; and he now informed them that the temperature of 55° C. which killed germs and vegetations in wine could be applied with equal success to vinegar after it was produced. The active germs of the mycoderma aceti were thus arrested at the right moment, the anguillulæ were killed and the vinegar remained pure and unaltered. “Nothing,” concluded Pasteur, “is more agreeable to a man who has made science his career than to increase the number of discoveries, but his cup of joy is full when the result of his observations is put to immediate practical use.”
This year 1867 marks a specially interesting period in Pasteur’s life. At Alais he had shown himself an incomparable observer, solely preoccupied with the silkworm disease, thinking, speaking of nothing else. He would rise long before anyone else so as to begin earlier the study of the experiments he had started, and would give his thought and attention to some detail for hours at a time. After this minute observation he would suddenly display a marvellous ingenuity in varying tests, foreseeing and avoiding causes of error, and at last, after so many efforts, a clear and decisive experiment would come, as it had done in the cases of spontaneous generation and of ferments.
The contrasts in his mind had their parallel in his character: this usually thoughtful, almost dreamy man, absorbed in one idea, suddenly revealed himself a man of action if provoked by some erroneous newspaper report or some illogical statement, and especially when he heard of some unscrupulous silkworm seed merchant sowing ruin in poor magnaneries for the sake of a paltry gain. When, on his return to Paris, he found himself mixed up with the small revolution in the Ecole Normale, he was seen to efface himself modestly before his masters when honours and titles came in question. Now he had interrupted his researches in order to do a kindness to the people of Orleans, who, practical as they were, and perhaps a little disdainful of laboratory theories, had been surprised to find him as careful of the smallest detail as they themselves were.