198 STUDY OF THE PHYSIOLOGY OF BACTERIA 



has constructed a device which answers every purpose for a 

 small class. The diagram, Fig. 141, explains it. 



The thermal death-point is determined by exposing the 

 organisms in thin tubes of broth at varying temperatures 

 for ten-minute periods and then plating out to determine 



Fig. 141. — Diagram of fittings for a cold incubator. 1, small tanl^^for 

 constant head, about 1 foot in each dimension, a, inflow; b, overflow; 

 c, lead pipe. 2, refrigerator, a', ice; b', flat coil under ice; c', outflow 

 to incubator. 3, incubator, a", cold water inflow; b", overflow; ther- 

 mometer and burner omitted. The diagram explains the construction. 

 The water cooled to about 14° with artificial ice by fiowing through the 

 lead coil under the ice, flows into the incubator which may be heated and 

 regulated in the usual way. 



growth. The effect of heat may also be determined by 

 exposing at a given temperature, e. </., 60°, for varying 

 lengths of time and plating out. 



B. Oxygen relations — whether the organism is aerobic, 

 anaerobic, or facultative is determined by inoculation in 



