38 CULTURE MEDIA. 
the habit of collecting it in this way, and, after the serum has sepa- 
rated, of drawing it off in little flasks having a long neck, as shown 
in Fig. 14. The neck of the flask, previously sterilized by heat, is 
shpped into the Woulf’s bottle beside the cotton plug, the bulb (a) 
having been previously gently heated to expand the contained air. 
As the heated air cools a partial vacuum is formed and the clear 
serum mounts into the little flask. One after another is filled in 
this way, and each one is hermetically sealed in the flame of a lamp 
Fia. 14. Fia. 15. ihe, 16. 
as soon as it is withdrawn. The sterile blood serum may be pre- 
served indefinitely in this way, and may be used as a liquid culture 
medium in the little flask, or it may be transferred to a test tube 
and solidified by heat whenever a solid blood-serum medium is re- 
quired. The advantage of preserving blood serum and other liquid 
media in these little flasks is in the fact that they may be preserved 
indefinitely without becoming contaminated or drying up, and that 
they are easily transported, while a liquid medium in a test tube 
must be kept upright. The contents of one of these flasks are readily 
