PREPARATION OF CULTURE MEDIA. 13 



advantage to place a shallow layer of a solution of 

 perchloride of mercury (or other non-volatile antiseptic) 

 in the bottom of each jar to prevent the medium from 

 drying up. It is scarcely necessary to add that not the 

 smallest trace of the lotion should be allowed to come 

 into contact with the cotton-wool plug of the tubes. 

 Or the medium may be kept from drying up by covering 

 the tubes with india-rubber caps sold for the pur- 

 pose. 



BROTH is very easily made, and as it is the foundation 

 of many other media, the practitioner is strongly advised 

 to prepare it for himself. 



Requisites : i. Liebig's extract of meat. 



2. Peptone. 



3. Common salt. 



4. A dilute solution of sodium carbonate about i 

 per cent., but the exact strength does not matter. 



5. A large flask, a stirring rod, and a large glass 

 funnel. 



6. Test-tubes and cotton-wool plugs. The exact size 

 of the tubes is unimportant, but 6 in. X in. is 

 convenient. The plugs are best prepared from wool 

 which has been previously sterilised by dry heat, and 

 should be fairly firm. The tube with the plug in situ 

 must be sterilised by dry heat ready for use. 



7. Litmus paper. 



Method. Take i litre of tap-water in the flask and 

 add 5 grammes of Liebig's extract, 10 grammes of 

 peptone, and 5 grammes of common salt, and boil until 

 all are dissolved. Test the reaction by withdrawing a 

 drop of the fluid on the stirring-rod and applying it to a 

 piece of litmus paper. You will find that it is slightly 

 acid. Now add some of the solution of soda drop by 

 drop, testing after each addition, until the reaction of 



