40 LIVING MATTER OR PROTOPLASM. 



But this law is true only within certain limits. Extreme heat 

 and cold are alike inimical to life, and as the temperature ap- 

 proaches these extremes all forms of vital action gradually or 

 suddenly cease. The limits are so variable, that it is not at pres- 

 ent possible to formulate any exact law which shall include all 

 known cases. For instance, many organisms are killed at the 

 freezing-point of water (0 C.) ; but certain forms of life have 

 withstood a temperature of --87 C. (123 F.), and recent ex- 

 periments show that frogs and rabbits may be chilled to an unex- 

 pected degree without fatal results. 



The upper limit is also inconstant, though less so than the 

 lower. Most organisms are destroyed at the point of boiling 

 water (100 C.), but the spores of fungi have been exposed to a 

 much higher temperature without* destruction (120-125 CA 

 As a rule, protoplasm is killed by a temperature varying from 40 

 to 50 C., the immediate cause of death being apparently due to 

 a sudden coagulation (p. 37) of certain substances in the proto- 

 plasm. Thus, if a brainless frog be gradually heated, death en- 

 sues at about 40 C., and the body becomes stiff and rigid (riyor 

 caloris) from the coagulation of the muscle-substance. The 

 lower forms of animal life agree well with plants in their " fatal 

 temperatures," which in most cases lie between 40 and 50 C. 



Lastly, it appears to be true that there is a certain most favor- 

 able or optimum temperature for the protoplasm of each spe- 

 cies of plant and animal, this optimum differing considerably in 

 different species. Probably the highest limit occurs among the 

 birds, where the uniform temperature of the body may be as 

 high as 40 C. The lowest occurs among the marine plants and 

 animals of the Arctic seas, or of great depths, where the tempera- 

 ture seldom rises more than a degree or two above the freezing- 

 point. Between these limits there appears to be great variation, 

 but 35 C. may be taken as the average optimum. 



Moisture. Protoplasm always contains a large amount of 

 water, of which indeed the lifeless portion of living things chiefly 

 consists. (See table on p. 35.) All plants and animals are killed 

 by complete drying, though some of the simpler forms resist 

 partial drying for a long time, becoming quiescent and reviving 

 a<rain when moistened, sometimes even after the lapse of years. 

 Hence water appears to be an essential constituent of protoplasm, 

 though, as in the case of mineral matters, we do not know the 



