As was natural in a pupil of Dumas, Balard, and Delafosse, Pasteur's first important piece of work was chemical and crystallographic, and we may best understand its spirit by recalling the work of Delafosse's master in mineralogy, the Abbé Hauy, who is still remembered for that bold attempt to visualize the ultimate structure of the crystal, to penetrate the inmost secret of its architecture, which also reappears in another way in the work of Mendelejeff. Pasteur's puzzle concerned the tartrates and paratartrates of soda and ammonia. These two salts are alike in chemical composition, in crystalline form, in specific gravity, and so on, but they differ in behaviour. Thus, as Biot had shown, a solution of tartrate deflects the plane of polarized light passed through it, while a solution of the paratartrate does not. The salts are the same, yet they behave differently. A note to the Academy from the famous chemist Mitscherlich emphasized the entire similarity of the two salts, and this acted as an additional stimulus to Pasteur. He succeeded in distinguishing the minute facets which even Mitscherlich had missed; he proved that the paratartrate is a combination of a left-handed and a right-handed tartrate, and did much else which only the expert chemist could duly explain. Biot was first doubtful, then delighted; Arago, who had also busied himself with these matters, moved that Pasteur's paper be printed in the memoirs of the Academy, and Mitscherlich himself congratulated the young discoverer who had tripped him up.
Already, then, in this minute and laborious piece of work, we may detect ultra-microscopic mental vision, and that rigorous accuracy so characteristic of the man. Yet it is interesting to observe that at this early stage he was sowing his wild oats of speculation. Impressed by the strange rotation of the plane of polarization exhibited by these organic salts, he deduced therefrom an hypothesis of molecular dissymmetry, and hazarded the view that this was a fundamental distinction between the organic and the inorganic. For various reasons, neither chemist nor biologist would nowadays accept this distinction; but it is hard to tell what Pasteur might have made of this inquiry had not circumstances, regretted at the time, directed his attention to very different subjects.
Being thus known in connection with tartrates, Pasteur was one day consulted, so the story goes, by a German manufacturer of chemicals, who was puzzled by the fermentation of his commercial tartrate of lime, which contained some admixture of organic impurities. Pasteur undertook to look into the matter, and probably deriving some hint from the previous work of Cagniard Latour, and Schwann who had demonstrated the yeast-plant which causes alcoholic fermentation, he demonstrated the micro-organism which fermented the tartrate of lime. He extended this discovery to other tartrates, and made the neat experiment of showing how the common blue mould (Penicillium glaucum), sown in paratartrate of ammonia, uses up all the right-handed tartrate and leaves the left-handed salt alone, its identical chemical composition notwithstanding. These and similar inquiries led him to tackle the whole question of fermentation, but his transference to Lille had probably much to do with this. For, as one of the chief industries of the district is making alcohol from beet-root and grain, Pasteur's practical sense led him to devote some of his lectures to fermentation; here, as always, as his biographer reminds us, wishful to make himself directly useful to his hearers.
The prevalent theory of fermentation, before Pasteur took the subject in hand, was that of Willis and Stahl, revised and elaborated by Liebig. According to this theory, nitrogenous substances in a state of decomposition upset the molecular equilibrium of fermentable matter with which they are in contact. What Pasteur did was to show that lactic, butyric, acetic, and some other fermentations, were due to the vital activity of micro-organisms. In spite of Liebig's prolonged opposition, Pasteur carried his point; and although some of his detailed interpretations have since been revised, it is universally admitted that he changed the whole complexion of the fermentation problem. It must, of course, be borne in mind that his theory of the vital nature of many fermentations does not apply to soluble ferments or enzymes—such as diastase and pepsin—which are chemical substances, not living organisms. Part, indeed, of the opposition to Pasteur's views was due to the fact that this distinction between organized and unorganized ferments was not at the time clearly drawn. Perhaps, indeed, we are as yet by no means out of the woods.
In the course of his work on fermentation, Pasteur made an important theoretical step by distinguishing the micro-organisms which require the presence of free oxygen, from forms which are able to live apart from free oxygen, obtaining what they require by splitting up oxygen-containing compounds in the surrounding medium. These he termed ærobic and anærobic respectively. Practically, this piece of work immediately led to what is known as the Orleans process of making vinegar. Some years later, after he had returned to Paris, he followed this up by his studies on wine, in the course of which he tracked various wine-diseases to their sources, and showed how deterioration might be prevented by raising the wine for a minute to a temperature of 50°C. The wine-tasters of Paris gave their verdict in his favour.
The old notion of spontaneous generation still lingered in some quarters, and in 1858 Pouchet had given new life to the question by claiming before the Academy of Sciences that he had succeeded in proving the origin of microscopic organisms apart from pre-existing germs. But Pasteur knew more than Pouchet as to the insidious ways of germs: he showed the weak point of his antagonist's experiments, and gained the prize, offered in 1860 by the Academy, for “well-contrived experiments to throw new light upon the question of spontaneous generation.” As every one knows, the victory was with Pasteur, but the idea is an old and recurrent one, and dies hard. Thus, not many years afterward, Pasteur and Tyndall had to fight the battle over again with Bastian. The important result of what seems at first sight an abstract discussion has been not only an increased knowledge of the distribution and dissemination of bacteria, but the establishment of the fundamental conditions and methods of experimental bacteriology....
[Here follows substantially the same narrative as that given by Sir J. Risdon Bennett on pages 36 to 49 of this volume. It recites how Pasteur devised preventives for the disease which was destroying the silkworms; preventives for splenic fever or anthrax.]
Opposition was an ever-recurrent factor in Pasteur's life. He had to fight for his crystallographic and chemical theories, and for his fermentation theory; he had to fight against the theory of spontaneous generation, and for his practice of inoculating as a preventive against splenic fever; he had to fight for each step. But no part of his work has met with so much opposition and adverse criticism as that concerning hydrophobia, though it is easy to exaggerate the importance of the discussion, in which Pasteur himself took little part. Feeling ran high in this country; hence, when it was announced that Pasteur—surely best qualified to speak—was to write the article Hydrophobia in “Chamber's Encyclopædia,” a shower of letters inundated the office; hence the article in question includes an editorially demanded summary of the grounds of the opposition by one of ourselves, and to which therefore we may refer the reader.
While avoiding controversy and partisanship as far as may be, the question remains, What did Pasteur do in regard to hydrophobia? His claims are to have proved, first of all, that the disease was particularly associated with the nervous system. The virus is usually spread through the saliva, but it is not found in the blood or lymph, and it has its special seat in the nerves, brain, and spinal cord. Secondly, he showed that the virus might be attenuated in its virulence. The spinal cord of a rabbit which has died of rabies, is, when fresh, powerfully virulent, but when exposed for a couple of weeks to dry air at a constant temperature of 23°-24°C. it loses its virulence. Thirdly, he showed that inoculation with the attenuated virus rendered an animal immune from infection with rabies. To make the animal immune it has first to be inoculated with infected spinal cord fourteen days old, then with that of thirteen days, and so on till inoculation with almost freshly infected spinal cord is possible. In this way the animal becomes refractory to the infection, and if it be bitten it will not die. Fourthly, he showed that even if the organism had been bitten, it was still possible to save it, unless the wounds were near the head—that is, within close reach of the central nervous system. For in the case of a superficial wound, say on hand or leg, the virus takes some considerable time to spread, and during this period of spreading and incubation it is possible to forestall the virus by inoculation with that which has been attenuated. In this case there is obvious truth in the proverb, “He gives twice who gives quickly.” And the outcome was, that while out of a hundred persons bitten, nineteen or twenty will in ordinary circumstances die, “the mortality among cases treated at the Pasteur Institute has fallen to less than 1-2 per cent.” According to another set of statistics, a mortality of 40 per cent. has been reduced to 1.3 per cent.; and of 1673 patients treated by Pasteur's method only thirteen died.
As to the adverse criticism of Pasteur's inoculation against rabies, it consists, first and second, of the general argument of the anti-vaccinationists, and thirdly, of specific objections. To the two former the school of Pasteur, of course, replies that the value of human life answers the one, and the results of experience the other; but on these controversies we cannot enter here. The main specific objections we take to be three—that as the micro-organism of rabies has not really been seen, the theory and practice of Pasteur's anti-rabic method lack that stability which is desirable; that the statistics in favor of the Pasteur procedure have been insufficiently criticised; that there have been failures and casualties, sometimes of a tragic nature. In regard to this last point—that deaths have occurred as the result of the supposed cure, instead of from the original infection—we may note that the possibility of such casualties was admitted by the English Investigation Committee (1887), while, on the other hand, Dr. Armand Ruffer, who speaks with much authority, denies with all deliberateness that there is any known case in which death followed as the result of Pasteur's treatment.