8 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



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 inches by f inch is convenient. 

 The plugs are best prepared from wool which has been pre- 

 viously sterilized by dry heat, and should be fairly firm. The 

 tube, with the plug in situ, must be sterilized 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 probably 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 the fluid is slightly alkaline.* Boil the 

 fluid for half an hour to coagulate any albumen which may be 

 present. 



Next filter the broth into a sterile flask, passing it through 

 a double thickness of white filter- or blotting-paper, and plug 

 the flask firmly with sterilized cotton-w 7 ool. 



If the broth is to be used for the manufacture of gelatin or 

 agar, it is next sterilized in the flask; while if it is to be used 

 as it is as a culture medium, it is decanted into tubes and then 

 sterilized. 



In decanting" media into tubes be very careful not to get the 

 plug wet, and not to let any of the medium get on to the upper 

 part of the tube; otherwise the plug will stick to the tube, and 

 there will be some danger of bacteria from the air " growing 

 through " the fluid contained in the interstices of the plug and 

 contaminating the culture. Ordinary non-absorbent (brown) 

 wool is better than the w r hite absorbent wool, as it is less easily 

 wetted. 



The broth (and solid culture media after being' melted) may 

 be poured into the tubes in the following way : A sterilized 



* If during the neutralizing process too much alkali is added, then it is 

 necessary to reacidify with dilute hydrochloric acid and reneutralize. The 

 sodium chloride formed makes no practical difference in the medium. 



