170 LOUIS PASTEUE. 



is added to it, in order that the whole mass of the 

 wort should be invaded as soon as possible after its 

 cooling by the alcoholic ferment alone the only one, 

 properly speaking, which can produce beer. If this 

 wort were treated in the same way as the must of 

 the grape, if it were abandoned to fermentation with- 

 out yeast to so-called spontaneous fermentation 

 this would hardly ever be purely alcoholic, as in the 

 must of grapes, which is protected by its acidity. 

 Most frequently, instead of beer, an acid or putrid 

 liquid would be obtained. Divers fermentations 

 would simultaneously take place in it. When the 

 wort has fermented and the beer is made, there is 

 still the fear of its rapid deterioration, which necessi- 

 tates its being quickly consumed. This condition 

 is sometimes disastrous to those employed in the 

 beer trade ; and the improvements in the manufac- 

 ture of beer which have been made during the last 

 forty years have all had for their object the removal of 

 this necessity for the daily production, so to speak, of 

 an article of which the consumption is liable to con- 

 stant variations. 



Formerly only one kind of beer was known, the 

 beer of high fermentation. The wort, after having 

 undergone cooling in the troughs, is collected in a 

 large open vat at a temperature of 20, and yeast is 

 added to it. When the fermentation begins to show 

 itself on the surface of the liquid, by the formation of 



