TEMPERATURE 



299 



certainty kills a watery suspension of the organisms in 

 question after an exposure of 10 minutes. 



Apparatus Required: 



Water-bath. For the purpose of observing the thermal death- 

 point a special water-bath is necessary. The temperature of this 

 piece of apparatus is controlled by means of a capsule regulator 



FIG. 155. Hearson's water-bath. 



that can be adjusted for intervals of half a degree centigrade 

 through a range of 30, from 50 C. to 80 C. by means of a spring, 

 actuated by the handle a, which increases the pressure in the 

 interior of the capsule. A hole is provided for the reception of 

 the nozzle of a blast pump, so that a current of air may be blown 

 through the water while the bath is in use, and thus ensure a 

 uniform temperature of its contents. Through a second hole is 

 suspended a certified centigrade thermometer, the bulb of which 

 although completely immersed in the water is raised at least 2 cm. 

 above the floor of the bath. 



Sterile glass capsules. 



Flask containing 250 c.c. sterile normal saline solution. 



Case of sterile pipettes, 10 c.c. (in tenths of a cubic centimetre). 



Special platinum loop. 



Test-tubes, 18 by 1.5 cm., of thin German glass. 



Case of sterile petri dishes. 



Tubes of agar or gelatine. 



