THE METHODS OF CULTIVATING MICROBES 51 



Liebig's extract (5 to 1000) and Cibil's extract of 

 beef (20 to 1000) may also be used for the same 

 purposes as bouillon; but these extracts require 

 very careful sterilisation by Professor Tyndall's 

 method of discontinuous heating. 



Liquid blood serum is used in drop-cultivations, 

 etc. It is obtained by collecting the blood of a 

 healthy sheep, calf, or horse, in sterilised flasks or 

 glass cylinders with stoppers. The vessel or vessels 

 containing the blood are placed in an ice-box or in 

 ice-cold water for about twenty-four hours, when 

 the separation of the clot will be completed. The 

 fluid serum is then transferred, by sterilised pipettes 

 (see Fig. 1 7 G), into sterilised test-tubes provided 

 with cotton-wool plugs. The test-tubes and their 

 contents are then heated in a serum steriliser for an 

 hour or two at 60 C. on six successive days. Up 

 to this point the serum forms a fluid medium ; but 

 in the majority of cases blood serum is used as a 

 solid medium. To solidify it, the serum (contained 

 in test-tubes, watch-glasses, or capsules) is placed 

 in an inspissator, kept at a temperature between 65 

 and 68 C., until solidification takes place. 



Milk is also used as a fluid medium. It is best 

 sterilised at 120C. in a steam digester or an auto- 

 clave. By this means it is readily sterilised in 

 about fifteen minutes. Milk can also be sterilised 

 in the steam steriliser at 100C., but it is necessary 

 to heat it for an hour on the first day, and for thirty 

 minutes on each of the following two days, that is 

 (unless an autoclave is used), milk must be sterilised 

 by discontinuous heating. 



