440 M. H. JACOBS 



end. Drops of water containing Paramecium could be placed in 

 it and removed with a capillary pipette. It was found by the 

 insertion of a thermocouple in such a drop of water and another 

 in a vessel of water at 41°C. into which the tube was plunged, 

 that the water in the tube reached approximately the tempera- 

 ture of that in the surrounding vessel in about twenty seconds. 

 This fact being known, a number of animals were placed in it 

 and plunged into water at 41° for two minutes and twenty 

 seconds (giving therefore an exposure of two minutes at 41°). 

 On removal, it was found that all were fatally injured. Another 

 lot were placed in the same tube, but they were brought at a 

 uniform rate from room temperature to 41° in two minutes and 

 then kept at exactly 41° for two minutes longer. In this case 

 about one-quarter of the individuals recovered. A third lot 

 were treated in the -same way except that in this case the rise 

 to 41° occupied twelve minutes. About one-half of these animals 

 recovered. Doubtless a slower increase of temperature would 

 have given even a higher percentage of recoveries. With starfish 

 larvae, it may be mentioned, that experiments made in the 

 same manner showed in every case exactly the reverse effect, 

 i.e., the slower the rate of temperature increase, the higher 

 the mortality. 



A considerable number of other experiments were tried with 

 Paramecium with the same general results as those mentioned. 

 The degree to which a slow change increased the final resistance 

 varied considerably with different races and under different ex- 

 perimental conditions, but in all cases it was very appreciable. 

 It is apparent, therefore, that in the case of this organism, at 

 least, the upper thermal death points that will be obtained in 

 different experiments by the methods usually employed may be 

 expected to be subject to considerable variations, which in many 

 cases will be difficult to predict. To what extent the same prin- 

 ciple will be found to apply in the case of other organisms can be 

 determined only by further observations. In any event, however, 

 the author believes that to be of value, data on the upper thermal 

 death points of organisms must include not only the length of 



