PROTOZOAN differences: resistance to heat 133 



from permanent death rigor. The method finally adopted was 

 this: A blood serum oven was used, and on the floor of this 

 oven a number of small glass dishes were arranged and numbered 

 consecutively. Each dish was 5 cm. square and 1 cm. thick 

 and was provided with a hollow ground cavity having a capacity 

 of about 3 cc. In each of these dishes was placed four or five 

 drops of the culture medium containing the species to be exam- 

 ined. Each dish was covered with a thin glass plate to prevent 

 evaporation, and the oven itself was closed in the usual way. 

 Using a medium flame the temperature was raised gradually from 

 the room temperature (ca. 20°) to 42° in about one hour. With 

 a little practice it was possible to bring about this rise in approxi- 

 mately the same time in different experiments; the few minutes 

 variation had no appreciable effect on the results. Not only 

 was there an advantage in making the rise of temperature a 

 very gradual one but each dish was in direct contact with the 

 copper floor of the oven, thus making the loss of heat by radia- 

 tion quite low. In order to determine as nearly as possible the 

 temperature of the dishes, a thermometer was placed in a hori- 

 zontal position on the floor of the oven. The thermometer, like 

 the dishes, was not only in contact with the metal upon which 

 it rested but was bathed in the warm air of the oven. Dr. E. 

 F. Phillips, of the United States Department of Agriculture, was 

 kind enough to make a series of thermo-electric determinations, 

 and by introducing a thermo-electric couple underneath the glass 

 cover of the dish, has made possible a comparison of the read- 

 ings of the thermometer with the real temperature of the culture 

 fluid containing the animals. In the region between 22° and 

 43° the actual temperature of the fluid was never more than 

 half a degree below that shown by the thermometer. Another 

 advantage of the method used was that the dishes could be 

 removed and allowed to cool before counting, thus giving an 

 opportunity for the recovery of those animals only temporarily 

 affected. In all these experiments at least half-an-hour was 

 allowed* for possible recovery. The dishes were removed from 

 the oven as successively higher temperatures were reached. Thus 

 dish no. 1 was removed when it had reached 35°, no. 2 at 36°, 



