“All ideas of holidays must be postponed; we must set to work in Jura as well as in Paris,” wrote Pasteur to his assistants. Bouley, who thought that the goal was reached, did not hide from himself the difficulties of interpretation of the alleged fact. He obtained from the Minister of Agriculture permission to try at Alfort this so-called vaccinal liquid on twenty sheep. “Yesterday,” wrote Pasteur to his son-in-law on August 13, “I went to give M. Chamberland instructions so that I may verify as soon as possible the Toussaint fact, which I will only believe when I have seen it, seen it with my own eyes. I am having twenty sheep bought, and I hope to be satisfied as to the exactitude of this really extraordinary observation in about three weeks’ time. Nature may have mystified M. Toussaint, though his assertions seem to attest the existence of a very interesting fact.” Toussaint’s assertion had been hasty, and Pasteur was not long in clearing up that point. The temperature of 55° C. prolonged for ten minutes was not sufficient to kill the bacteridia in the blood; they were but weakened and retarded in their development; even after fifteen minutes’ exposure to the heat, there was but a numbness of the bacteridium. Whilst these experiments were being pursued in the Jura and in the laboratory of the Ecole Normale, the Alfort sheep were giving Bouley great anxiety. One died of charbon one day after inoculation, three two days later. The others were so ill that M. Nocard wanted to sacrifice one in order to proceed to immediate necropsy; Bouley apprehended a complete disaster. But the sixteen remaining sheep recovered gradually and became ready for the counter test of charbon inoculation. Whilst Pasteur was noting the decisive points, he heard from Bouley and from Roux at the same time, that Toussaint now obtained his vaccinal liquid, no longer by the action of heat, but by the measured action of carbolic acid on splenic fever blood. The interpretation by weakening remained the same. “What ought we to conclude from that result?” wrote Bouley to Pasteur. “It is evident that Toussaint does not vaccinate as he thought, with a liquid destitute of bacteridia, since he gives charbon with that liquid; but that he uses a liquid in which the power of the bacteridium is reduced by the diminished number and the attenuated activity. His vaccine must then only be charbon liquid of which the intensity of action may be weakened to the point of not being mortal to a certain number of susceptible animals receiving it. But it may be a most treacherous vaccine, in that it might be capable of recuperating its power with time. The Alfort experiment makes it probable that the vaccine tested at Toulouse and found to be harmless, had acquired in the lapse of twelve days before it was tried at Alfort, a greater intensity, because the bacteridium, numbed for a time by carbolic acid, had had time to awaken and to swarm, in spite of the acid.”