174 LOUIS PASTEUR. 



preservative ? But beer is a drink necessarily charged 

 with carbonic acid, and the application of heat to con- 

 siderable masses of the liquid would expel this gas. 

 It would be a very complicated business to attempt to 

 preserve this gas, or to introduce it afresh after it had 

 been expelled. This difficulty does not arise when the 

 beer is bottled. At a temperature of 50 to 55, 

 the process of heating not only cannot take away 

 from the beer all its carbonic acid, but it does not 

 prevent the secondary fermentation from taking place 

 to a certain extent, and this allows of the beer being 

 heated immediately after it is put into bottles. This 

 heating of the beer is practised on a large scale in 

 Europe and in America. In honour of Pasteur the 

 process is called Pasteurisation, and the beer Pas- 

 teurised beer. 



But Pasteur was not content with simply destroying 

 the ferments of these diseases, he wished above all to 

 prevent their introduction. At the moment when 

 the wort is raised to the boiling-point, when the 

 germs of disease are destroyed by the heat, if the 

 cooling of the wort is effected in contact with both 

 air and yeast free from exterior germs, the beer may 

 be made under conditions of exceptional purity. Some 

 brewers, taking for their basis Pasteur's principles, con- 

 structed an apparatus which enabled them to protect 

 the wort while it was cooling from the organisms of 

 the air, and to ferment this wort with a leaven as 



