INTRODUCTION 



UNTIL about twenty-five years ago it was generally 

 supposed that some organic substances in the act of 

 undergoing decomposition are capable of causing the 

 alteration and decay of other organic substances with 

 which they are placed in contact, and it was by the 

 assumption of such communicated decomposition from 

 one substance to another that Liebig sought to explain 

 the various well-known phenomena of fermentation. 

 Thus Liebig conceived of the ordinary alcoholic fer- 

 mentation of sugar as being brought about, not by the 

 living and growing yeast-cells, but, on the contrary, by 

 the dead yeast undergoing decomposition. As long as 

 this theory was the accepted doctrine of the day, it was 

 not surprising to find chemists attaching great import- 

 ance to the organic matter in water which analyses re- 

 vealed, and which was known to have been derived 

 from decomposing vegetable and animal substances 

 with which the water had been in contact. It was not 

 unnaturally supposed that such decomposing organic 

 matters, if present in drinking water, would tend to set 

 up putrefactive and other injurious changes in the 

 digestive organs with which they were brought in con- 

 tact. But the theory, or rather do^ma, of fermentation 



