158 BACTERIOLOGY. 



then be raised slowly to the boiling point, and the liquid 

 stirred constantly. By following the directions given in 

 the preparation of the meat extract it will not be neces- 

 sary to resort to clarification with egg-albumin. If the 

 gelatin fails to solidify on cooling it is because it has been 

 heated too long. In that case more gelatin should be added 

 (50 g.), the liquid then neutralized and treated as before. 



If the filtered gelatin answers the above requirements 

 it is ready to be "tubed", that is, filled into sterile tubes. 

 This should be done by means of a small glass funnel ! and 

 the tubes should be filled to a depth of 1^ inches. The 

 large tubes will then contain about 8 c.c. of gelatin. The 

 utmost care should be taken to avoid touching the neck of 

 the tube with the gelatin, since otherwise the cotton will 

 adhere to the tube. The tubes, filled with gelatin, are 

 placed in the'copper sterilizing pail (Fig. 26, p. 164). 



The tubes employed are 15 or 16 mm. in diameter and 

 150 mm. long. Although the tubes may be new and ap- 

 parently clean, they should be invariably washed with hot 

 water, preferably slightly acidulated, in order to remove any 

 alkali that may remain from the manufacture. They should 

 then be rinsed in cold water, and allowed to drain till dry. 

 The tubes are then plugged with cotton. The simplest way 

 of doing this is to place a piece of cotton, 1^ to 2 inches 

 square, on the mouth of the tube and then pushing down 

 the middle by means of a narrow glass tube, rod, or match- 

 stick. The best and most solid plugs are obtained, by fold- 

 ing over a piece of cotton into thirds and the rolling up 

 from the end. A firm, cylindrical plug is thus obtained 

 without any frayed out border. The plug should be 1 to 1| 

 inches long and should project out of the tube as little as 

 possible. The nutrient medium can be filled into the tubes 

 which have not been sterilized. The subsequent exposure 

 to steam will bring about sterilization of the walls of the 



1 The globe funnel shown in Fig-. 72 can be used for the rapid filling 

 of tubes. 



