GELATIN AND POTATO MEDIA. 159 



tube, of the plug, and of the contents. It is customary, 

 however, to subject the tubes, before they are filled with 

 culture media, to sterilization by means of dry -heat. It is 

 desirable to do this owing to the presence of highly resist- 

 ant spores in the cotton, or in the tubes. The tubes are 

 placed erect in baskets made of heavy wire gauze. 



Dry-heat Sterilization. 



By sterilization is meant the total destruction of all 

 living forms, in or about the object exposed to the process. 

 Heat is the most effective agent in securing this condition. 

 It may be used as dry-heat or as moist-heat. Bach of 

 these has its special advantages. Dry-heat is used only, 

 for the sterilization of glass and metal instruments or 

 articles. Owing to the high temperature employed, 

 150-170°, culture media and other organic substances cannot 

 be thus treated, since charring would promptly result. For 

 such substances moist, steam-heat is usually employed. 



The dry-heat sterilizer, or oven, is made of sheet-iron and is 

 double-walled, thus permitting the circulation of hot air around the 

 inner compartment. The heat is applied by means of a good Bunsen 

 burner. As usually constructed, the heat is directed against the 

 bottom of the inner compartment. In time, this bottom burns out 

 and permits the flame to enter. The apparatus, in that case, cannot 

 be readily repaired. This defect in construction can be readily 

 obviated by attaching, to the bottom, a loose plate, which is held in 

 position by a pin, or nut. This plate, when burned out, can be 

 replaced at a merely nominal expense. The sterilizer can be still 

 further improved by placing the two openings, which are on top, in 

 the rear corners. Through each of these openings is passed a brass 

 tube, freely perforated on all sides (see Fig. 25). These tubes are 

 intended to receive a thermometer and a thermo-regulator, respect- 

 ively. These instruments are thus protected against accidental 

 injury and do not interfere with the placing in, or the taking out of 

 baskets and other apparatus. Pig. 25 shows a dry-heat sterilizer 

 which, however, is twice the usual length. The thermo-regulator in- 



