ADDRESS. 7 
mentation. The prevailing opinion regarding this class of phenomena 
when they first engaged his attention was that they were occasioned 
primarily by the oxygen of the air acting upon unstable animal or vege- 
table products, which, 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 microscopic fungus 
which grows as the sweetwort ferments ; and he had attributed the break- 
ing 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 fermentation were staple articles of manufacture, determined to 
study that process thoroughly ; and as a result he became firmly con- 
vinced 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 fibrin, 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 fibrin, but steadily 
increasing as the fermentation proceeded. Struck by the analogy pre- 
sented 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 minuteness 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 fibrin 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 fibrin or other animal material altogether, 
and substituting for it mineral salts containing the requisite chemical 
elements. A trace of the grey deposit being applied 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 way. 
I have referred to this research in some detail because it illustrates 
