STERILIZATION BY HOT AIR 91 



always be given to the condition of the copper bottom; for 

 the direct application of heat to the sheet-iron plate upon 

 which the substances to be sterilized stand results not only 

 in destruction of the apparatus, but frequently in destruc- 

 tion of the substances undergoing sterilization. 



Since the temperatiu-e at which this form of sterihzation 

 is usually accomplished is high, from 150° to 180° C, .it is 

 well to have the apparatus encased in asbestos boards, to 

 diminish the radiation of heat from its surfaces. This not 

 only confines the heat to the apparatus, but guards against 

 the destructive action of the radiated heat on woodMork, 

 furniture, etc., that may be in the neighborhood. 



Thermal Death-point of Bacteria. — By "thermal death- 

 point of bacteria" is meant the temperature necessary to 

 kill them in a given time. As this varies with different 

 species, it is an aid to identification. For the practical pur- 

 poses of the sanitarian the knowledge is of fundamental 

 importance. The thermal death-point of an organism is 

 ascertained by subjecting it to varying degrees of tempera- 

 ture for five or ten minutes until the point is reached where 

 it is killed. The test is best carried out by means of small 

 glass bulbs, the so-called Sternberg bulbs, or through the 

 use of capillary tubes containing a small amount of fluid 

 inoculated with the organism to be studied. The bulb, 

 or tube, is sealed in the gas flame and placed in a water- 

 bath kept at 50° C. for five minutes. ' Sub-cultures are now 

 made to learn whether the bacteria have been killed or not. 

 If the organism survives the test is repeated at 55°, 60°, 65 °, 

 and 70° C. Finally, the test is repeated for each degree of 

 temperature between the points where growth is still apparent 

 and where the organisms have been kiUed. If the bacteria 

 were killed when heated to 60° C. for fi^'e minutes, but sur- 



