THERMAL DEATH-POINT 755 



possible ; when disinfected they are removed by the 

 opposite door into the other room, thereby avoiding all 

 chance of reinfection. Steam at a pressure of about 20 Ib. 

 is admitted into the jacket and then passes to the inner 

 chamber, the object of the jacket being to warm the 

 chamber, and so prevent condensation. For the same 

 purpose hot air is sometimes injected beforehand to warm 

 the chamber and articles, and after the steam disinfection, 

 can again be injected for drying. The length of time 

 required for disinfection does not exceed a half to one 

 hour. 



In Thresh 's disinfector the steam is generated from a 

 saline solution (calcium chloride), which has a boiling- 

 point (105 C.) higher than that of water. 



The thermal death-point of a number of organisms in pure 

 culture has been determined by many investigators. Eyre 

 suggests the following as " standard conditions " for determining 

 thermal death-points : 



1. Length of " time exposure " to be ten minutes. 



2. Emulsion to be prepared from " optimum cultivation." 



3. The vehicle in which culture is suspended to be sterile salt 

 solution or sterile distilled water. 



4. Strength of emulsion to correspond to about 1 milligramme 

 of culture per cubic centimetre. 



o. Bulk of emulsion to be not less than 3 c.c. 



6. Emulsion to be contained in test-tube of 1-5 cm. diameter 

 with walls 1 mm. thick. 



7. Emulsion to be heated in a water-bath regulated by a 

 delicate thermo -regulator, with an accurate thermometer. 



8. Broth cultivations and agar plates both to be used in deter- 

 mining the death of the bacteria, and the period of observation of 

 these cultures to be extended, when necessary, to seven or four- 

 teen days. The experiments to be repeated at least once. 



9. Thermal death-point to be first roughly determined to 

 within 5 C. 



10. Thermal death-point to be finally determined to within 

 1C., and to be defined as that temperature which causes the 



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