14 MICRO-ORGANISMS AND FERMENTATION. 



3. Sterilisation of the Air is best attained, as stated 

 above, by means of cotton-wool filters ; sulphuric acid, salt 

 water baths, cloth filters, etc., are less efficient. In labora- 

 tories, where work must often be performed in germ-free air, 

 a glass chamber is employed, the front of which can be raised 

 sufficiently for the operator to introduce his hands. Some 

 time before using the chamber, the whole of its inner surface 

 must be washed, and the chamber then closed. The particles 

 and germs suspended in the air will then settle to the moist 

 bottom and remain there. 



4. DISINFECTION. 



Another method of killing disturbing germs is by the use 

 of disinfectants, which act as poisons on the micro-organisms. 

 Not a few of these substances have found application in 

 practice. The limit for the employment of such poisonous 

 substances must be determined for each individual case. As 

 manipulations with such poisons may be deleterious to the 

 operator, it is important to ascertain their proper degree of 

 dilution. 



Investigations having for their object the determination of 

 the poiver of resistance of the various species of micro- 

 organisms to poisons have proved that it varies greatly 

 in different cultures of one and the same species, not only 

 for the spores, but also for vegetative forms. A young 

 culture behaves differently from an old one, and the same 

 applies to individuals belonging to one and the same culture. 



cannot keep absolutely free from germs when the fermentation takes 

 place in open tuns, but a great deal can be effected in this direction by 

 acquiring a true and thorough understanding of the matter. ' The expert 

 brewer will always take care that the air in the fermenting room is 

 kept as free from germs as possible, and also that the tuns as well as all 

 utensils that are immersed into the fermenting liquid, e.g.. thermometers, 

 sample glasses, etc., are always perfectly clean. As a matter of course, 

 these precautionary measures could not acquire any real practical import- 

 ance until, through Hanseris reform, absolutely pure yeast had been 

 introduced into the fermenting room. 



