So thought the Society of French Agricultors, when it decided, on February 21, 1881, to offer to Pasteur a medal of honour. J. B. Dumas, detained at the Académie des Sciences, was unable to attend the meeting. He wrote to Bouley, who had been requested to enumerate Pasteur’s principal discoveries at that large meeting—“I had desired to make public by my presence my heartfelt concurrence in your admiration for him who will never be honoured to the full measure of his merits, of his services and of his passionate devotion to truth and to our country.”
On the following Monday, Bouley said to Dumas, as they were walking to the Académie des Sciences, “Your letter assures me of a small share of immortality.”
“See,” answered Dumas, pointing to Pasteur, who was preceding them, “there is he who will lead us both to immortality.”
On that Monday, February 28, Pasteur made his celebrated communication on the vaccine of splenic fever and the whole graduated scale of virulence. The secret of those returns to virulence lay entirely in some successive cultures through the body of certain animals. If a weakened bacteridium was inoculated into a guinea-pig a few days old it was harmless; but it killed a new-born guinea-pig.
“If we then go from one new-born guinea-pig to another,” said Pasteur, “by inoculation of the blood of the first to the second, from the second to a third, and so on, the virulence of the bacteridium—that is: its adaptability to development within the economy—becomes gradually strengthened. It becomes by degrees able to kill guinea-pigs three or four days old, then a week, a month, some years old, then sheep themselves; the bacteridium has returned to its original virulence. We may affirm, without hesitation, though we have not had the opportunity of testing the fact, that it would be capable of killing cows and horses; and it preserves that virulence indefinitely if nothing is done to attenuate it again.
“As to the microbe of chicken-cholera, when it has lost its power of action on hens, its virulence may be restored to it by applying it to small birds such as sparrows or canaries, which it kills immediately. Then by successive passages through the bodies of those animals, it gradually assumes again a virulence capable of manifesting itself anew on adult hens.
“Need I add, that, during that return to virulence, by the way, virus-vaccines can be prepared at every degree of virulence for the bacillus anthracis and for the chicken-cholera microbe.
“This question of the return to virulence is of the greatest interest for the etiology of contagious diseases.”
Since charbon does not recur, said Pasteur in the course of that communication, each of the charbon microbes attenuated in the laboratory constitutes a vaccine for the superior microbe. “What therefore is easier than to find in those successive virus, virus capable of giving splenic fever to sheep, cows and horses, without making them perish, and assuring them of ulterior immunity from the deadly disease? We have practised that operation on sheep with the greatest success. When the season comes for sheep-folding in the Beauce, we will try to apply it on a large scale.”
The means of doing this were given to Pasteur before long; assistance was offered to him by various people for various reasons; some desired to see a brilliant demonstration of the truth; others whispered their hopes of a signal failure. The promoter of one very large experiment was a Melun veterinary surgeon, M. Rossignol.