PREPARATION OF CULTURE MEDIA. 13 
advantage to place a shallow layer of a solution of 
perchloride of mercury (or other won-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. 
Brortu 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 :—1. Liebig’s extract of meat. 
2. Peptone. 
3. Common salt. 
4. A dilute solution of sodium carbonate—about 1 
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 2 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 i situ 
must be sterilised by dry heat ready for use. 
7. Litmus paper. 
Method.—Take 1 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 
