20 MICRO-ORGANISMS AND FERMENTATION. 



biological nature. The methods employed include the use 

 of a current of steam, steam under pressure, boiling in water 

 or on the sand bath, and the treatment may be prolonged 

 for a considerable period if it is desired to kill not only vegeta- 

 tive cells, but also spores. In either case it is obviously of 

 importance to take care that during the subsequent cooling 

 only sterile air is admitted to the vessel. This is secured in 

 the case of Pasteur flasks by the use of a tube with two bends, 

 in which any germs that are sucked in are deposited ; in the 

 case of Erlenmeyer and Freudenreich flasks, by sealing them 

 with cotton wool filters. Whilst the hopped wort commonly 

 used in zymophysiological laboratories will stand boiling on 

 the sand bath, and after a comparatively short boiling can be 

 preserved unchanged, wort gelatine and other gelatines cannot 

 stand treatment on a sand bath or such prolonged boiling on 

 a water bath or in steam, that will ensure the destruction of 

 ah 1 spores, because there is always a danger that after such 

 treatment the gelatine will no longer solidify at a temperature 

 of 25 C. The same difficulty is met with in the sterilisation 

 of the mash in distilleries and of wort in the air yeast factories, 

 owing to the great separation of albuminoid substances which 

 takes place at the boiling point, causing a complete change in 

 the character of the liquid. For this reason it is impossible 

 to apply all the results of experiments obtained with properly 

 sterilised liquids to the very different circumstances that 

 obtain in practice. In all such cases use is made of the method 

 of fractional or discontinuous sterilisation introduced by 

 Tyndall. Its object is to bring about the germination of 

 spores of bacteria and similar resistant organisms by main- 

 taining the material at a gentle heat for some time, so that 

 the cells may subsequently be killed at comparatively low 

 temperatures. The material is first warmed, perhaps to a 

 temperature of 70 C., or it may be heated for a quarter of an 

 hour in a current of steam in order to kill the vegetative cells. 

 It is then maintained at room temperature, or, better still, 

 at the most favourable temperature for the development of 

 spores (about 35 C.), and after the lapse of a day, or even of 

 a shorter period, when it is assumed that germination is 

 complete, the material is again heated. By repeated treat- 



