PREPARATION OF CULTURE MEDIA. 287 



the steamer for twenty minutes and filter. Then add neutral 

 litmus so as to produce a reddish-purple colour. Divide into 

 test-tubes and sterilise in the steamer on three successive days. 



Neutral Litmus. This solution is prepared by adding 300 c.c. 

 of rectified spirit to two ounces of commercial litmus. After 

 standing for a fortnight the spirit is poured off and a second 

 300 c.c. added, and allowed to stand for a second fortnight. 

 The alcohol is then poured off, and all traces of the spirit 

 removed by exhausting the flask with an air-pump. The litmus. 

 is now added to 800 c.c. of water placed in a flask and allowed 

 to stand for twenty-four hours. The watery extract so obtained 

 is filtered into a flask, and a few drops of pure sulphuric acid 

 added until the solution has an acid reaction. Baryta solution 

 is then added in excess, and the fluid left until it has an alkaline 

 reaction. The solution is then filtered, and carbon-dioxide 

 passed through it until an acid reaction is again obtained. The 

 Barium carbonate is filtered off, and the solution placed in a 

 test-tube. Sterilisation is effected in steam steriliser on three 

 successive days. 



Potato-gelatine. Crush 500 grammes of peeled potato and 

 macerate in a litre of water for three or four hours. Sift and 

 allow to stand for twenty-four hours, then decant, and make up 

 the volume to 1000 c.c. Dissolve 150 to 200 grammes of 

 gelatine by heat, and boil for a few minutes. Then add a solu- 

 tion of soda until the reaction is feebly but distinctly acid. 

 Heat to 115 C. for five minutes, and filter through " Chardin " 

 paper placed in a hot funnel. Divide into tubes and sterilise at 

 112-115 C. Just before use Eisner recommends the addi- 

 tion of 1 per cent, of potassium iodide to the gelatine previously 

 melted in the " plate bath/" 



Glucose-formate-broth. For the study of bacteria under 

 anaerobic conditions Kitasato recommended the addition of 0*4* 

 gramme of sodium formate and two grammes of grape sugar to 

 ordinary broth. Fakes'* medium for the isolation of B. coli and 

 B. typhosus is made as follows : " To ordinary meat iniusion 

 are added 1 per cent, peptone, 0*5 per cent, sodium chloride, 2 

 per cent, glucose, and 0'4 per cent, sodium formate. When 

 these have all been dissolved by heating the medium is neutral- 

 ised, and after neutralisation 4 c.c. of normal soda solution per 



