11 



point which coincides with the freezing innul. A precise 

 definition can be given of that death point, as well as in 

 the case in which death was supposed to result from a cool- 

 ing to a specific minimum. The solidification of a given 

 proportion of moisture takes place at a definite tempera- 

 ture. The time factor has little importance ; it is simply 

 related to the total mass of material to be killed in the 

 same way as the time necessary to melt a certain quantity 

 of metal depends on the amount present and bears no rela- 

 tion to the melting point. 



But an organism which is killed when left to congeal at 

 the freezing point might not be killed if brought rapidly 

 to some hundred of degrees below zero, where congelation 

 cannot take place on account of the high viscosity of the 

 material (cf. Luyet, 1937). Thus, temperatures below 

 the lethal point might be non-lethal. 



One and the same organism may, therefore, possess a 

 zone of lethal temperatures above zero, a sharp death 

 point slightly below zero, and a zone of non-lethal tem- 

 peratures some hundred degrees below zero. And these 

 are only a few^ of the many possibilities that one can 

 imagine. 



It is clear, then, that the notion of lethal temperature 

 depends on the idea that one has of the mechanism of 

 death and, therefore, is susceptible of multiple interpre- 

 tations. There being no possible general definition of the 

 death point, of the survival point, etc., we shall, in this 

 review, use these expressions in the sometimes unprecise 

 sense given to them by the various authors, without 

 attempting to define them more exactly. 



The most important factors to be considered in a study 

 of the temperatures at which animals or plants, organs, 

 tissues, cells, or protoplasm, die, are the following: 1. 

 The species or the type of material studied; 2. The ex- 

 ternal temperature to which the material has been sub- 

 jected; 3. The time of action of this temperature; 4. The 



