HEAT RESISTANCE OF PARAMECIUM 



213 



the other in an acid medium. The results will be described 

 separately and then compared. 



Paramecium caudatum in alkaline medium. The medium was 

 prepared by boiling 20 grams of hay in 500 cc. of tap-water for 

 five minutes. Two days later, i.e., about the time of maximum 

 acidity, just enough N/20 NaOH was added to render the fluid 

 neutral to litmus. The culture was then seeded with a single 

 individual isolated from a pure line previously grown in the 

 laboratory for some weeks. The animals developed rapidly 

 and the culture remained densely populated for about four 

 months. The medium soon became dark brown in color and 

 decidedly alkaline to litmus, and retained its alkaline reaction 

 throughout its history. The effect of NaCl solutions was tested 

 when the culture was about ten weeks old. Control experiments 

 were carried out in every case at the same time, using five drops 

 of the unchanged culture medium in each dish. The results 

 obtained with M/lOO NaCl are summarized in table 1. 



Experiments with M/50 NaCl were carried out when the cul- 

 ture was about three months old; the results are summarized in 

 table 2. 



TABLE 1 



Effects oj M/lOO NaCl on the heat resistance of P. caudatum from an alkaline 



medium 



MEANS OF THE ABOVE FATAL 

 TEMPERATURE ZONES 



/in M/lOONaCl 42.03° 



\ in controls 40 . 9° 



