56 APPLIED BACTERIOLOGY 



rubber caps can be at once put on, if the precaution is 

 - taken to insert a short bit of string between the cap and 

 the edge of the tube to allow for the expansion of the air ; 

 if this is not done, the caps would be blown off during the 

 sterilization. 



The capped tubes are now placed in the rack which fits 

 into the steam steriliser, and steamed for three successive 

 days for about fifteen to twenty minutes. After the last 

 sterilisation, some of the tubes, while the contents are 

 still liquid, are placed in a sloping position to allow the 

 gelatine to expose as much surface as possible for streak 

 cultures. 



(6) Carbolic Acid Gelatine.— To every 100 c.c. of the above 

 10 per cent, gelatine solution is added 4 c.c. of a 5 per cent, 

 solution of pure phenol. The tubes are then filled and 

 sterilised as above. This gelatine is used for separating 

 the typhoid bacillus or the Bacterium coli communis. 



(7) Grape-sugar Gelatine. — Two per cent, of glucose is 

 dissolved in the ordinary gelatine medium, and sterilised 

 as usual. 



(8) Agar-agar. — Twenty grammes of agar -agar* is finely 

 cut up, and soaked in a dilute solution of acetic acid (5 c.c. 

 glacial acid in 500 c.c. of water) for twenty minutes. This 

 causes the agar to swell up and become more readily 

 soluble. The agar is then thoroughly washed in water to 

 free it from all traces of acid, after which it is well boiled 

 with a litre of nutrient broth, prepared by the method 

 already described, for about thirty-five to forty-five minutes, 

 until all the agar has become quite dissolved ; the water 

 lost by evaporation is replaced from time to time. Care is 

 then taken to see that the medium is faintly alkaline, after 



* Agar-agar is a vegetable substance procured frora some species of 

 Japanese marine' algse. It is generally obtained commercially in the 

 form of long threads, which should be cut up as finely as possible. 



