172 CULTURE MEDIA 



the incubator at 37 C. and at the end of that time 

 eliminate any contaminated tubes. 



15. Store such tubes as remain sterile for future use. 



Milk.- . : 



1. Pour i litre of fresh cow's or goat's milk into a 

 large separating funnel, and heat in the steamer at 

 100 C. for one hour. 



2. Remove from the steamer and estimate the re- 

 action of the milk (normal cows' milk averages +17). 

 If of higher acidity than +20, or lower than + 10, re- 

 ject this sample of milk and proceed with another 

 supply of milk from a different source. 



Reject milk to which antiseptics have been added as 

 preservatives. 



3. Allow the milk to cool, when the fat or cream 

 will rise to the surface and form a thick layer. 



4. Draw off the subnatant fat-free milk into sterile 

 tubes (10 c.c. in each). 



5. Sterilise in the steamer at 100 C. for twenty 

 minutes on each of five successive days. 



6. Incubate at 37 C. for forty-eight hours and 

 eliminate any contaminated tubes. Store the re- 

 mainder for future use. 



Litmus Milk. 



1. Prepare milk as described above, sections i to 3. 



2. Draw off the subnatant fat-free milk into a flask. 



3. Add sterile litmus solution, sufficient to colour 

 the milk a deep lavender. 



4. Tube, sterilise, etc., as for milk. 



Nutrose Agar (Eyre). 



(This is a modification of the well known Drigalski- 

 Conradi medium originally introduced for the isolation 

 of B. typhosus). 



i. Collect 250 c.c. perfectly fresh ox serum (vide 



