had a fermentation theory of his own—pooh-poohed this idea altogether; maintaining the presence of the yeast-plant to be a mere concomitant, and refusing to believe that it had any real share in the process. But, in 1843, Professor Helmholtz, then a young, undistinguished man, devised a method of stopping the passage of organic germs from a fermenting into a fermentable liquid, without checking the passage of fluids; and, as no fermentation was then set up, he drew the inference that the "particulate" organic germs, not the soluble material of the yeast, furnish the primum mobile of this change—a doctrine which, though now universally accepted, had to fight its way for some time against the whole force of chemical authority.
A little before Cagniard de la Tour's discovery, a set of investigations had been made by Schulze and Schwann to determine whether the exclusion of air was absolutely necessary to prevent the appearance of living organisms in decomposing fluids, or whether these fluids might be kept free from animal or vegetable life by such means as would presumably destroy any germs which the air admitted to them might bring in from without, such as passing it through a red-hot tube or strong sulphuric acid. These experiments, it should be said, had reference rather to the question of "spontaneous generation," or "abiogenesis," than to the cause of fermentation and decomposition, its object being to determine whether the living things found by the microscope in a decomposing liquid exposed to the air spring from germs brought by the atmosphere or are generated de novo in the act of decay—the latter doctrine having then many upholders. But the discovery of the real nature of yeast and the recognition of the part it plays in alcoholic fermentation gave an entirely new value to Schulze's and Schwann's results, suggesting that putrefactive and other kinds of decomposition may be really due, not (as formerly supposed) to the action of atmospheric oxygen upon unstable organic compounds, but to a new arrangement of elements brought about by the development of germinal particles deposited from the atmosphere.
It was at this point that Pasteur took up the inquiry, and, for its subsequent complete working-out, science is mainly indebted to him; for, although other investigators—notably Professor Tyndall—have confirmed and extended his conclusions by ingenious variations on his mode of research, they would be the first to acknowledge that all those main positions which have now gained universal acceptance—save on the part of a few obstinate "irreconcilables" have been established by Pasteur's own labors. These positions may be briefly summarized as follows:
1. That no organic fluid undergoes spontaneous fermentation or
- It was, I remember, in or about that year that Professor Liebig's visit to England gave me the opportunity of showing him some yeast under a high power of the microscope. He said that he had not before seen its component cells so distinctly.