60 APPLIED BACTERIOLOGY 



with cotton-wool that has been heated in the hot-air 

 steriliser for about an hour at 120° C. The plugs, when 

 inserted, should be a little over an inch long, and should 

 fit as tightly as possible. The best way of plugging tubes 

 is to place a loose piece of wool on the mouth of the tube, 

 which is then pressed home with a penholder with a rotary 

 motion. The loose ends of the cotton-wool are then cut or 

 singed off, and each tube covered with tinfoil; or the 

 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 sterili- 

 sation. 



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 

 sterilized 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 dis- 

 solved 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 



* 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. 



