CHAPTER XIV. 

 TO DETERMINE THE THERMAL DEATH-POINT. 



SEVERAL methods may be employed for this purpose. 

 Roughly, it may be done by keeping a bouillon-culture of 

 the micro-organism to be studied in a water-bath whose 

 temperature is gradually increased from that of the body 



to 75 C. 



Into a fresh bouillon-culture thus exposed to heat, the 

 experimenter cautiously, and at given intervals, intro- 

 duces a platinum loop or a capillary pipette, and with- 

 draws a drop of the culture which he inoculates into 

 fresh bouillon and stands aside to grow. It is economy 

 to make the transplantations rather infrequently at first 

 and frequently later on in the experiment, when the tem- 

 perature is ascending. In an ordinary determination it 

 would be well to make a transfer at 40 C., one at 45 C., 

 another at 50, still another at 55, and then beginning at 

 60 make one for every additional degree up to 75 C. 

 or above. The day following the experiment it will be 

 observed that all the cultures grow except those heated 

 beyond a certain point, as 60 C. and upward, when it 

 can properly be concluded that 60 C. is the thermal 

 death-point. If all the transplantations grow, of course 

 the maximum temperature that the bacteria can endure 

 was not reached, and the experiment must be performed 

 again with higher temperatures. 



When more accurate information is desired, and one 

 wishes to know how long the micro-organism can endure 

 some such temperature as 60 C. without losing its vital- 

 ity, a dozen or more bouillon-tubes may be inoculated 

 with the germ to be studied, and stood in the water-bath 

 at the temperature to be investigated. The first can be 



176 



