Il6 PHYSIOLOGICAL CHEMISTRY. 



was noticed that a scum formed over the liquid and that a 

 small amount of this substance was capable of quickly inciting 

 similar fermentation in more saccharine liquid or wine. The 

 nature of this scum became in time the subject of microscopic 

 investigation (by Leuwenhock) and we have here probably 

 the beginning of our real study of ferments. It was in 1680 

 that Leuwenhock recognized that this scum in the case of 

 beer yeast consisted of minute globules with peculiar proper- 

 ties ; the full value of this discovery, however, was not gener- 

 ally admitted and more than a century passed before any great 

 advance was made by others. Lavoisier toward the end of 

 the eighteenth century gave the first explanation of the chem- 

 istry of alcoholic fermentation, as he was able to point out the 

 relation of carbon dioxide and alcohol to the parent sugar. 

 But the cause of the action was not much discussed arid just 

 what the function of the cells or globules of Leuwenhock is 

 remained obscure until the time of Pasteur. 



Before taking up the important work of Pasteur something 

 must be said of discoveries in other directions. The older 

 conception of fermentation was widened by the addition of 

 new facts. In 1780 Scheele isolated lactic acid from sour 

 milk and later investigators began to look for the agent re- 

 sponsible for the production of this acid. About 1848 the 

 probable nature of an organism which appeared to be always 

 associated with lactic fermentation was pointed out by Blon- 

 deau. Various formulas were given for the production of 

 lactic acid in quantity, but it often happened that the final 

 product was an entirely different substance, viz : butyric acid. 

 Butyric fermentation was therefore added to the list of these 

 peculiar reactions, and various speculations were advanced to 

 connect the different phenomena. Meanwhile the situation 

 became still further complicated by the gradual recognition of 

 a new group of reactions which exhibited many of the essen- 

 tial features of the alcoholic and acetic fermentations, and 

 which, therefore, of necessity were classed as ferment reac- 

 tions. Several chemists had observed the peculiar behavior 



