96 METHODS OF STERILISATION. 



compare the illustrated catalogues of such firms as make the 

 -supply of these appliances a speciality, e.g. C. Desaga of Heidelberg ; 

 Erhardt and Metzger of Darmstadt, &c. 



It is not our purpose now to give a detailed initiation into the 

 work of a fermentation physiologist's laboratory, but rather to 

 describe, in bold outline, only so much as is necessary to facilitate 

 the object of the present work, viz., the study of the character 

 and modes of action of the organisms of fermentation. 



72. Freeing 1 the Air from Germs. 



There are two chief methods by which liquid substances and 

 gases can be sterilised, viz., either by killing the germs present 

 therein, or by removing them by passing the liquid or gas through 

 & suitable filter. The sterilisation of air on a large scale is effected 

 exclusively by the latter method, the prototype of which was con- 

 stituted by the tubes, plugged with cotton-wool, first employed by 

 Schroder and Dusch. The air is, therefore, passed through a cotton- 

 wool filter, as it is termed, such a one being used, for example, 

 to purify the air admitted to the sterilised wort in an apparatus 

 for the pure cultivation of yeast. It will not be out of place to 

 lay stress on the fact that such a filter will only work efficiently 

 provided it be thoroughly dry; otherwise the Eumycetes spores 

 entangled therein will germinate and develop into long-thread 

 cells, which will penetrate right through the filter and quickly 

 form new spores, so that the air at the end of the filter nearest 

 the wort is not only not freed from germs, but is probably richer 

 therein than before. Attention to the air filters must, conse- 

 quently, not be neglected. E. CH. HANSEN (III.) has reported 

 on experiments made by Poulsen concerning the time during 

 which such filters continue, under normal conditions of practical 

 working, to pass the air in a germ-free state. 



The cotton-wool plugs with which, since the time of Schroder 

 And Dusch, it is customary to close test-tubes, bottles, and flasks 

 in which cultures of organisms or stores of nutrient media are 

 kept, are simply small cotton-wool filters. They are especially 

 brought into action when currents of air pass into the vessels as a 

 consequence of the partial vacua formed within them by a lower- 

 ing of temperature, the germs in which are retained by the plugs. 

 The efficiency of the filter depends on its being kept dry. Its 

 reliability is not, however, permanent, since, though the fission 

 fungi are always retained, this is not the case with the spores of 

 mould fungi, which are so abundantly met with in the air. These 

 latter are very troublesome, as they often produce much mischief 

 even when the mycologist has taken the greatest care. If the room 

 in which the cultures are kept be free from moisture, then the cul- 

 tures dry up very rapidly, which, in order to preserve their 

 vitality, necessitates their being frequently re-inoculated into fresh 



