48 PASTEUR 



answers were made to them, but no answer that 

 was scientifically satisfactory. 



The alchemists of the middle ages thought 

 that yeast had a certain power of transmuta- 

 tion and that fermentation, if applied to metals, 

 would enable them to transmute a base metal, 

 such as iron, into a precious metal, such as 

 gold. The first of all to approach the truth was 

 Paracelsus, who compared fermentations to 

 diseases, but his idea was still vague, and not 

 based upon experiments. We must wait until 

 we come down to Lavoisier in order to see fer- 

 mentations studied upon a basis of facts, but 

 neither this great chemist nor those who fol- 

 lowed him, Gay-Lussac, Cagniard-Latour, 

 Schwann, Helmholtz, Liebig, succeeded in dem- 

 onstrating their real origin. The theory most 

 generally accepted, at the time when Pasteur 

 began his researches, was that of Liebig, who 

 attributed fermentations to matter in the course 

 of decomposition, which played the role of a 

 ferment in the mediums into which they were 

 introduced. 



