62 



LABORATORY EXERCISES IN BACTERIOLOGY. 



ing the protruding end of the stopper; however, the cotton should not be too closely 

 packed into the tube, and should have been rolled into an even cylinder rather than 

 loosely and unevenly crowded into the tube. Ordinarily common cotton batting is 

 used for the purpose ; the only objection, however, to absorbent cotton is its cost. Bat- 

 ting will assume, with proper sterilizing heat to which the tubes are subsequently ex- 

 posed, a light brown hue which may be taken as a rough indicator of proper sterilization ; 



FIG. 1 6. TYPES OF CULTURE FLASKS. 



however, if higher temperatures should accidentally be attained, batting will present a 

 disadvantage in that a dark oil is driven from the cotton, which will enter the tubes as a 

 vapor and condense upon the inner surface. If permitted to remain it is apt to interfere 

 with the development of inoculations, is unsightly, and should be removed by thorough 

 washing. 



Sterilization of Tubes. After the tubes have been cleaned and the cotton stoppers 



adjusted, they are placed in a basket 

 made of wire netting (Fig. 12), and steril- 

 ized by baking in the dry-air oven for an 

 hour at a temperature of 140 to 150 C. 

 (v. Ex. 7 and 8). It is not essential that 

 the complete fractional heating should be 

 carried out in this preliminary process (ex- 

 cept in special cases, as when the tubes 

 are intended for blood-serum), since after 

 the introduction of the media the thorough 

 sterilization of the latter as well as of the 

 tube will be necessary. 



Special Forms. In preparing tubes for potatoes, if the tubes be of the common 

 form, it is well, before adjustment of the cotton stoppers, to place in the interior of each 

 a small bit of glass tube or rod or a wad of cotton to serve as a support for the potato 

 cylinder in order to prevent the latter from descending to the bottom of the tube, where 

 in the process of sterilization there will collect more or less liquid. This liquid is re- 

 tained in the tube to afford moisture to colonies of bacteria subsequently grown upon 

 the surface of the potato. A special form of tube is sometimes used to obviate the need 



FIG. 17. TYPES OF DISTRIBUTION FLASKS. 



