AND THE HEALING ART 493 



air acting upon unstable animal or vegetable products, whicli, breaking up under 

 its influence, communicated disturbance to other organic materials in their 

 vicinity, and thus led to their decomposition. Cagniard-Latour had indeed 

 shown several years before that yeast consists essentially of the cells of a micro- 

 scopic fungus which grows as the sweetwort ferments ; and he had attributed 

 the breaking up of the sugar into alcohol and carbonic acid to the growth of 

 the micro-organism. In Germany, Schwann, who independently discovered 

 the yeast plant, had published very striking experiments in support of analogous 

 ideas regarding the putrefaction of meat. Such views had also found other 

 advocates, but they had become utterly discredited, largely through the great 

 authority of Liebig, who bitterly opposed them. 



Pasteur, having been appointed as a young man Dean of the Faculty of 

 Sciences in the University of Lille, a town where the products of alcoholic fer- 

 mentation were staple articles of manufacture, determined to study that process 

 thoroughly ; and as a result he became firmly convinced of the correctness of 

 Cagniard-Latour' s views regarding it. In the case of other fermentations, 

 however, nothing fairly comparable to the formation of yeast had till then 

 been observed. This was now done by Pasteur for that fermentation in which 

 sugar is resolved into lactic acid. This lactic fermentation was at that time 

 brought about by adding some animal substance, such as fibrine, to a solution 

 of sugar, together with chalk that should combine with the acid as it was formed. 

 Pasteur saw, what had never before been noticed, that a fine grey deposit was 

 formed, differing little in appearance from the decomposing fibrine, but steadily 

 increasing as the fermentation proceeded. Struck by the analogy presented 

 by the increasing deposit to the growth of yeast in sweetwort, he examined it 

 with the microscope, and found it to consist of minute particles of uniform size. 

 Pasteur was not a biologist, but although these particles were of extreme minute- 

 ness in comparison with the constituents of the yeast plant, he felt convinced 

 that they were of an analogous nature, the cells of a tiny microscopic fungus. 

 This he regarded as the essential ferment, the fibrine or other so-called ferment 

 serving, as he believed, merely the purpose of supplying to the growing plant 

 certain chemical ingredients essential to its nutrition not contained in the sugar. 

 And the correctness of this view he confirmed in a very striking manner, by 

 doing away with the fibrine or other animal material altogether, and substituting 

 for it mineral salts containing the requisite chemical elements. A trace of the 

 grey deposit being api)lied to a solution of sugar containing these salts in addition 

 to the chalk, a brisker lactic fermentation ensued than could be procured in 

 the ordinary wa}'. 



I have referred to this research in some detail because it illustrates 



