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.