The grasp of Pasteur on this class of subjects was embracing. He studied acetic fermentation, and found it to be the work of a minute fungus, the mycoderma aceti, which, requiring free oxygen for its nutrition, overspreads the surface of the fermenting liquid. By the alcoholic ferment the sugar of the grape-juice is transformed into carbonic acid gas and alcohol, the former exhaling, the latter remaining in the wine. By the mycoderma aceti the wine is, in its turn, converted into vinegar. Of the experiments made in connection with this subject one deserves especial mention. It is that in which Pasteur suppressed all albuminous matters, and carried on the fermentation with purely crystallisable substances. He studied the deterioration of vinegar, revealed its cause, and the means of preventing it. He defined the part played by the little eel-like organisms which sometimes swarm in vinegar casks, and ended by introducing important ameliorations and improvements in the manufacture of vinegar. The discussion with Liebig and other minor discussions of a similar nature, which M. Radot has somewhat strongly emphasized, I will not here dwell upon.
It was impossible for an inquirer like Pasteur to evade the question—Whence come these minute organisms which are demonstrably capable of producing effects on which vast industries are built and on which whole populations depend for occupation and sustenance? He thus found himself face to face with the question of spontaneous generation, to which the researches of Pouchet had just given fresh interest. Trained as Pasteur was in the experimental sciences, he had an immense advantage over Pouchet, whose culture was derived from the sciences of observation. One by one the statements and experiments of Pouchet were explained or overthrown, and the doctrine of spontaneous generation remained discredited until it was revived with ardour, ability, and, for a time, with success, by Dr. Bastian.
A remark of M. Radot's on [page 103] needs some qualification. 'The great interest of Pasteur's method consists,' he says, 'in its proving unanswerably that the origin of life in infusions which have been heated to the boiling point is solely due to the solid particles suspended in the air.' This means that living germs cannot exist in the liquid when once raised to a temperature of 212° Fahr. No doubt a great number of organisms collapse at this temperature; some indeed, as M. Pasteur has shown, are destroyed at a temperature 90° below the boiling point. But this is by no means universally the case. The spores of the hay-bacillus, for example, have, in numerous instances, successfully resisted the boiling temperature for one, two, three, four hours; while in one instance eight hours' continuous boiling failed to sterilise an infusion of desiccated hay. The knowledge of this fact caused me a little anxiety some years ago when a meeting was projected between M. Pasteur and Dr. Bastian. For though, in regard to the main question, I knew that the upholder of spontaneous generation could not win, on the particular issue touching the death temperature he might have come off victor.
The manufacture and maladies of wine next occupied Pasteur's attention. He had, in fact, got the key to this whole series of problems, and he knew how to use it. Each of the disorders of wine was traced to its specific organism, which, acting as a ferment, produced substances the reverse of agreeable to the palate. By the simplest of devices, Pasteur, at a stroke, abolished the causes of wine disease. Fortunately the foreign organisms which, if unchecked, destroy the best red wines are extremely sensitive to heat. A temperature of 50° Cent. (122° Fahr.) suffices to kill them. Bottled wines once raised to this temperature, for a single minute, are secured from subsequent deterioration. The wines suffer in no degree from exposure to this temperature. The manner in which Pasteur proved this, by invoking the judgment of the wine-tasters of Paris, is as amusing as it is interesting.
Moved by the entreaty of his master, the illustrious Dumas, Pasteur took up the investigation of the diseases of silkworms at a time when the silk-husbandry of France was in a state of ruin. In doing so he did not, as might appear, entirely forsake his former line of research. Previous investigators had got so far as to discover vibratory corpuscles in the blood of the diseased worms, and with such corpuscles Pasteur had already made himself intimately acquainted. He was therefore to some extent at home in this new investigation. The calamity was appalling, all the efforts made to stay the plague having proved futile. In June 1865 Pasteur betook himself to the scene of the epidemic, and at once commenced his observations. On the evening of his arrival he had already discovered the corpuscles, and shown them to others. Acquainted as he was with the work of living ferments, his mind was prepared to see in the corpuscles the cause of the epidemic. He followed them through all the phases of the insect's life—through the eggs, through the worm, through the chrysalis, through the moth. He proved that the germ of the malady might be present in the eggs and escape detection. In the worm also it might elude microscopic examination. But in the moth it reached a development so distinct as to render its recognition immediate. From healthy moths healthy eggs were sure to spring; from healthy eggs healthy worms; from healthy worms fine cocoons: so that the problem of the restoration to France of its silk-husbandry reduced itself to the separation of the healthy from the unhealthy moths, the rejection of the latter, and the exclusive employment of the eggs of the former. M. Radot describes how this is now done on the largest scale, with the most satisfactory results.
The bearing of this investigation on the parasitic theory of communicable diseases was thus illustrated: Worms were infected by permitting them to feed for a single meal on leaves over which corpusculous matter had been spread; they were infected by inoculation, and it was shown how they infected each other by the wounds and scratches of their own claws. By the association of healthy with diseased worms, the infection was communicated to the former. Infection at a distance was also produced by the wafting of the corpuscles through the air. The various modes in which communicable diseases are diffused among human populations were illustrated by Pasteur's treatment of the silkworms. 'It was no hypothetical, infected medium—no problematical pythogenic gas—that killed the worms. It was a definite organism.'[5] The disease thus far described is that called pébrine, which was the principal scourge at the time. Another formidable malady was also prevalent, called flacherie, the cause of which, and the mode of dealing with it, were also pointed out by Pasteur.
Overstrained by years of labour in this field, Pasteur was smitten with paralysis in October 1868. But this calamity did not prevent him from making a journey to Alais in January 1869, for the express purpose of combating the criticisms to which his labours had been subjected. Pasteur is combustible, and contradiction readily stirs him into flame. No scientific man now living has fought so many battles as he. To enable him to render his experiments decisive, the French Emperor placed a villa at his disposal near Trieste, where silkworm culture had been carried on for some time at a loss. The success here is described as marvellous; the sale of cocoons giving to the villa a net profit of twenty-six millions of francs.[6] From the Imperial villa M. Pasteur addressed to me a letter, a portion of which I have already published. It may perhaps prove usefully suggestive to our Indian or Colonial authorities if I reproduce it here:—