62 



MANUAL OF BACTERIOLOGY. 



closed, and the pressure begins to rise. After leaving the 

 articles to be sterilized in the autoclave for the length of time 

 desired, the apparatus must not be opened while the steam 

 contained within it is still under pressure, as there may be a 

 sudden evolution of steam upon the removal of the pressure 

 which may blow the media out of their tubes and flasks. On 

 the other hand, the pressure must not be allowed to drop below 

 zero, for in this case the plugs of the 

 tubes or flasks may be sucked in. The 

 apparatus must, therefore, be kept closed 

 until the gauge shows that the atmos- 

 pheric pressure is as great as the pressure 

 within, or, what is equivalent, until the 

 temperature has fallen to ioo° C. but 

 not below this temperature. A good 

 rule is to watch until the pressure is 

 very near zero, and the temperature 

 very near ioo° C, and then to cautiously 

 open the stop-cock a very little. The 

 autoclave may be opened as soon as the 

 hissing from the out-driven steam ceases. 

 Gelatin may be damaged by sterilization 

 with the autoclave, if it be heated too 

 long or too high a temperature. Media 

 containing sugar should not be sterilized 

 in the autoclave (see page 67). 

 Sterilization by Filtration. — Ordinary filters are useless 

 for this purpose, but the tubes or bougies of unglazed porcelain 

 devised by Pasteur and Chamberland are effective when prop- 

 erly employed. They are made in several different grades of 

 porosity. In the Berkefeld filter bougies made of infusorial 

 earth are used, and the pores in this are larger than those of the 

 Pasteur filter. The coarser of these filters permits the passage 

 of very small bacteria. Bacteria of average size, like bacillus 



Fig. 16. — Kitasato 

 filter. 



