THERMAL DEATH-POINT. ■ 517 



DRY HEAT. 



The ordinary dry heat sterilizer is employed in study- 

 ing the action of this form of heat on bacteria. The bulb 

 of the thermometer should be placed so as to be on a level 

 with the objects exposed. It should not, however, rest on 

 a metal surface. A thermometer is used to maintain a con- 

 stant temperature. 



The suspensions prepared as above cannot obviously be 

 employed as such. It is customary to soak, in these sus- 

 pensions, bits of sterile silk threads or small sqtiares of 

 muslin. These are then allowed to dry before they are 

 uspd. Sterile cover-glasses are also smeared on one side 

 with the suspended bacteria, and when dry they can be used 

 in a similar manner. 



The silk threads are prepared by cutting up some of 

 the silk in lengths of about 1. 5 cm. These should be placed 

 in a plugged test-tube and sterilized in the dry-heat oven. 

 At the same time some muslin is cut up into squares of 

 about 1 cm. on a side. These are sterilized in a Petri dish. 

 Thoroughly clean cover-glasses (p. 140), 18 to 20 mm. square 

 are cut into halves by means of a ruler and diamond. The 

 oblong slips of glass are likewise placed in a Petri dish and 

 sterilized. 



The bacterial suspension prepared as above (p. 514) is 

 transferred to a sterile wide Esmarch dish and the proper 

 number of silk threads and muslin squares are then added 

 and are allowed to soak thoroughly. One by one, they are 

 then picked up by means of sterile forceps, and arranged in 

 rows in. a sterile Petri dish. A corresponding number of 

 the sterile cover-glasses are likewise placed in the Petri 

 dish and each one is covered with a large loopful of the sus- 

 pension. This is spread over as much of the surface as pos- 

 sible and care is taken that the liquid does not run over the 

 edge to the under side of the cover-glass. The silk-threads, 

 muslin squares and cover-glasses are then allowed to dry 



