Nisard wrote to him (June 19): “My dear friend, I heard of your loss, and I sympathize most cordially with you.... Take all the time necessary to you. You are away in the service of science, probably of humanity. Everything will be done according to your precise indications. I foresee no difficulty ... everything is going on well at the Ecole. In spite of your reserve—which is a part of your talent—I see that you are on the track, as M. Biot would have said, and that you will have your prey. Your name will stand next to that of Olivier de Serres in the annals of sericiculture.”
On his return to Alais Pasteur went back to his observations with his scientific ardour and his customary generous eagerness to lighten the burden of others. He wrote in the introduction to his Studies on Silkworm Disease the following heartfelt lines—
“A traveller coming back to the Cévennes mountains after an absence of fifteen years would be saddened to see the change wrought in that countryside within such a short time. Formerly he might have seen robust men breaking up the rock to build terraces against the side and up to the summit of each mountain; then planting mulberry trees on these terraces. These men, in spite of their hard work, were then bright and happy, for ease and contentment reigned in their homes.
“Now the mulberry plantations are abandoned, the ‘golden tree’ no longer enriches the country, faces once beaming with health and good humour are now sad and drawn. Distress and hunger have succeeded to comfort and happiness.”
Pasteur thought with sorrow of the sufferings of the Cévenol populations. The scientific problem was narrowing itself down. Faced by the contradictory facts that one successful set of cocoons had produced corpuscled moths, while an apparently unsuccessful set of worms showed neither corpuscles nor spots, he had awaited the last period of these worms with an impatient curiosity. He saw, amongst those which had started spinning, some which as yet showed no spots and no corpuscles. But corpuscles were abundant in the chrysalides, those especially which were in full maturity, on the eve of becoming moths; and none of the moths were free from them. Perhaps the fact that the disease appeared in the chrysalis and moth only explained the failures of succeeding series. “It was a mistake,” wrote Pasteur (June 26, 1865), “to look for the symptom, the corpuscle, exclusively in the eggs or the worms; either might carry in themselves the germ of the disease, without presenting distinct and microscopically visible corpuscles.” The evil developed itself chiefly in the chrysalides and the moths, it was there that it should chiefly be sought. There should be an infallible means of procuring healthy seed by having recourse to moths free from corpuscles.
This idea was like a searchlight flashed into the darkness. Pasteur thus formulated his hypothesis: “Every moth containing corpuscles must give birth to diseased seed. If a moth only has a few corpuscles, its eggs will provide worms without any, or which will only develop them towards the end of their life. If the moth is much infected, the disease will show itself in the earliest stages of the worm, either by corpuscles or by other unhealthy symptoms.”
Pasteur studied hundreds of moths under the microscope. Nearly all, two or three couples excepted, were corpuscled, but that restricted quantity was increased by a precious gift. Two people, who had heard Pasteur ventilate his theories, brought him five moths born of a local race of silkworms and nurtured in the small neighbouring town of Anduze in the Turkish fashion, i.e. without any of the usual precautions consisting in keeping the worms in nurseries heated at an equal temperature. Everything having been tried, this system had also had its turn, without any appreciable success. By a fortunate circumstance, four out of those five moths were healthy.
Pasteur looked forward to the study in comparisons that the following spring would bring when worms were hatched both from the healthy and the diseased seed. In the meanwhile, only a few of the Alaisians, including M. Pagès, the Mayor, and M. de Lachadenède, really felt any confidence in these results. Most of the other silkworm cultivators were disposed to criticize everything, without having the patience to wait for results. They expressed much regret that the Government should choose a “mere chemist” for those investigations instead of some zoologist or silkworm cultivator. Pasteur only said, “Have patience.”
He returned to Paris, where fresh sorrow awaited him: Camille, his youngest child, only two years old, was seriously ill. He watched over her night after night, spending his days at his task in the laboratory, and returning in the evening to the bedside of his dying child. During that same period he was asked for an article on Lavoisier by J. B. Dumas, who had been requested by the Government to publish his works.
“No one,” wrote Dumas to Pasteur—“has read Lavoisier with more attention than you have; no one can judge of him better.... The chance which caused me to be born before you has placed me in communication with surroundings and with men in whom I have found the ideas and feelings which have guided me in this work. But, had it been yours, I should have allowed no one else to be the first in drawing the world’s attention to it. It is from this motive, also from a certain conformity of tastes and of principles which has long made you dear to me, that I now ask you to give up a few hours to Lavoisier.”