Anyone but Pasteur would have been staggered, but he was not the man to allow himself to be discouraged by à priori opinions, and by assertions which were more or less guesswork. He was resolved not to abandon his preconceived idea until experiment had pronounced upon it with precision. All scientific research, in order to be undertaken and followed up with success, should have, as point of departure, a preconceived idea, an hypothesis which we must seek to verify by experiment. To judge of the value of the facts which Pasteur had just announced, it was necessary to know if there existed the relation of cause and effect between the corpuscles and the disease. This was the great point to be elucidated.
But if, without preliminary groping, he had discovered the way to be pursued, Pasteur subsequently brought to bear his rare prudence as an experimentalist. For five years he returned annually for some months to Alais. The little house nestling among the trees called Pont-Guisquet became at the same time his habitation and his silkworm nursery. It is hemmed in by mountains, up the sides of which terraces rise, one above the other, planted with mulberry trees. The solitude was profound. Madame Pasteur and her daughter constituted themselves silkworm-rearers—performing their part in earnest, not only gathering the leaves of the mulberry trees, but also taking part in all the experiments. The assistants of the École Normale, Duclaux, Maillot, Gernez, and Raulin, grouped themselves around their master. Thus, in an out-of-the-way corner of the Cévennes was formed a colony seeking with ardour the solution of an obscure problem, and the means of curing or preventing a disease which had for so long a time blighted one of the great sources of the national wealth.
One of the first cares of Pasteur was to settle the question as to the contagion of the disease. Many hypotheses had been formed regarding this contagion, but few experiments had been made, and none of them were decisive. Opinions were also very much divided. Some considered that contagion was certain; the majority, however, either doubted or denied its existence; some considered it as accidental. It was said, for example, that the evil was not contagious by itself, but that it became so through the presence and complication of other diseases which were themselves contagious. This hypothesis was convenient, and it enabled contradictory facts to be explained. If some persons had seen healthy worms, which had been mixed up either by mistake or intention with sickly ones, perish, and if they insisted on contagion, others forthwith replied by diametrically opposite observations.
But whatever the divergences of opinion might be, everyone at all events believed in the existence of a poisonous medium rendered epidemic by some occult influence. Pasteur soon succeeded, by accurate experiments, in proving absolutely that the evil was contagious.
One of the first experiments was as follows. After their first moulting, he took some very sound worms free from corpuscles, and fed them with corpusculous matter, which he prepared in the following simple manner. He pounded up a silkworm in a little water, and passed a paint-brush dipped in this liquid over the whole surface of the leaves. During several days there was not the least appearance of disease in the worms fed on those leaves; they reached their second moulting at the same time as the standard worms which had not been infected. The second moulting was accomplished without any drawback. This was a proof that all the worms, those infected as well as the standard lot, had taken the same amount of nourishment. The parasite was apparently not present. Matters remained in this state for some days longer. Even the third moulting was got through without any marked difference between the two groups of worms. But soon important changes set in. The corpuscles, which had hitherto only showed themselves in the integuments of the intestines, began to appear in the other organs. From the second day following the third moulting—that is to say, the twelfth after the infection—a visible inequality distinguished the infected from the non-infected worms. Those of the standard lot were clearly in much the best health. On examining the infected worms through a magnifying glass, a multitude of little spots were discovered on their heads, and on the rings of their bodies, which had not before shown themselves. These spots appeared on the exterior skin when the interior skin of the intestinal canal contained a considerable number of corpuscles. It was these corpuscles that impeded the digestive functions, and interfered with the assimilation of the food. Hence arose the inequality of size of the worms. After the fourth moulting, the same type of disease was noticed as that which was breaking out everywhere in the silkworm nurseries, especially the symptom of spots on the skin, which had led to the disease being called pébrine. The peasants said that the worms were peppered. The majority of the worms were full of corpuscles. Those which spun their cocoons produced chrysalides which were nothing but corpusculous pulp, if such a term be allowed.
It was thus proved that the corpuscles, introduced into the intestinal canal at the same time as the food of the worms, convey the infection into the intestinal canal, and progressively into all the tissues. The malady had in certain cases a long period of incubation, since it was only on the twelfth day that it became perceptible. Finally, the spots of pébrine on the skin, far from being the disease itself, were but the effect of the corpuscles developed in the interior; they were but a sign, already removed from the true seat of the evil. 'If these spots of pébrine,' thought Pasteur, 'were considered in conjunction with certain human maladies in which spots and irruptions appear on the body, what interesting inductions might present themselves to minds prepared to receive them!'
Pasteur was never tired of repeating this curious experiment, or of varying its conditions. Sometimes he introduced the corpusculous food into healthy worms at their birth, sometimes at the second or third moulting. Occasionally, when the worms were about to spin their cocoons, the corpusculous food was given them. All the disasters that were known to have happened in the silkworm nurseries, their extent and their varied forms, were faithfully reproduced. Pasteur created at will any required manifestation of pébrine. When he infected quite healthy worms, after their fourth moulting, with fresh corpusculous matter, these worms, even after several meals of corpusculous leaves alternated with meals of wholesome leaves, made their cocoons. It might have been supposed that in this case the contagion had not taken effect. This was but a deceptive appearance. The communication of the disease exhibited itself in a marked degree in the chrysalides and in the moths. Many of the chrysalides died before they turned into moths, and their bodies might be said to be entirely composed of corpuscles. Such moths as were formed, and which emerged from their cocoons, had a most miserable appearance. The disease sometimes went so far as to render breeding and the laying of eggs impossible.
Faithful to the rules prescribed by the experimental method, Pasteur was careful to reproduce these same experiments with the worms of the standard lot, from which all infected worms had been selected. He fed these healthy worms on leaves over which a clear infusion made from the remains of moths or worms exempt from corpuscles had been spread with a paint-brush, instead of leaves contaminated with corpusculous remains. This food kept the worms in their usual health. Could there be a better proof that the corpuscles alone were the real cause of the pébrine disease?
These experiments, I repeat, threw a strong light on the nature of the disease, and exactly accounted for what took place in the industrial cultivations. From the malady which attacked the worms at their birth, decimating a whole cultivation, down to the invisible disease that may be said to inclose itself in the cocoon, all was now explained. One of the effects of the plague which had most excited the surprise and thwarted the efforts of cultivators was the impossibility of finding productive eggs, even when they tried to obtain them from the cocoons of groups which had succeeded perfectly well as far as the production of cocoons was concerned. It was proved that almost invariably the following year the eggs of these fine-looking groups were unproductive. Numbers of the agricultural boards, and practitioners, not being able to believe in the existence of the disease in collections that were so satisfactory as regards the abundance and beauty of the cocoons, persisted in thinking that the failures had an origin not connected with the seed itself. This resulted in deception after deception, often even in mistakes that were much to be regretted. Frequently the best husbandmen were known to reserve for the production of eggs some very fine cultivations, not having observed in the worms either spots of pébrine or corpuscles even up to the time when the mounting of the brambles had been accomplished; and the year following they had the pain of seeing all the cultivations from these eggs perish. These circumstances, so well calculated to produce discouragement and to give the disease a mysterious character, met with their natural explanation in the facts proved by experimental infection.