246 THE POPULAR SCIENCE MONTHLY. 



had a fermentation theory of his own pooh-poohed this idea alto- 

 gether ; 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, undistin- 

 guished 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 sol- 

 uble 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 investiga- 

 tions 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 sup- 

 posed) 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 atmos- 

 phere. 



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 sjiontaneous 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 micro- 

 scope, lie said that he had not before seen its component cells so distinctly. 



