PREPARA TION OF NUTRIENT MEDIA. 41 



filled. If by accident tlie pipette touches any object, 

 or if inadvertently it is laid down, it must be set aside 

 for tbe time, and another sterilised pipette must be 

 taken. Above all, everything must be done as quickly 

 and skilfully as possible, the operator continually 

 bearing in mind that to the surface of all unsterilised 

 objects innumerable bacteria are clinging, which the 

 slightest disturbance may shake on to the sterilised 

 object, thus rendering it unfit for use. In conse- 

 quence, it is absolutely necessary to thoroughly wash 

 one's hands in corrosive sublimate solution (1 : 1000), 

 in order to free them from all germs, before filling the 

 test-tubes. 



The test-tubes containing the blood serum may be 

 now subjected to "fractional or intermittent sterili- 

 sation," by exposing them for an hour a day for eight 

 days to a temperature of 58° 0. This is most con- 

 veniently accomplished in an incubator, which is de- 

 scribed later, and in which the temperature can be very 

 accurately regulated and kept constant. The blood 

 serum may be sterilised in a similar manner after it 

 has solidified ; but this is not to be recommended, as it 

 loses in transparency and also parts with a great deal 

 of the moisture it contains during the operation. This 

 method is not really certain to free the blood serum 

 from germs, as there are bacteria which, even in the 

 vegetative condition, are quite able to withstand this 

 temperature. Although a relatively successful result 



