38 PROTOPLASM AND THE CELL. 



benumb our own bodies (provided they become really chilled), and 

 in lower animals the heart beats more slowly, the movements be- 

 come sluggish or cease, breathing becomes slow and heavy, --in 

 a word, all of the vital actions become depressed, --whenever 

 the ordinary temperature is sufficiently lowered. If we chill 

 the rotating protoplasm of Clmi'tt, or Nitella,, the vibrating cilia 

 of ciliated cells, or an actively flowing Amoeba, the movements 

 become slower, and finally cease altogether. 



On the other hand, moderate warmth favors protoplasmic 

 action. Benumbed fingers become once more nimble before the 

 warmth of the fire. In a hot room the frog's heart beats more 

 rapidly, cilia lash more energetically, the Amoeba flows more 

 rapidly, and the protoplasm of Chara courses more swiftly. In 

 the winter months the protoplasm of plants and of many animals 

 is in a state of comparative inactivity. Most plants lose their 

 leaves and stop growing ; many animals bury themselves in the 

 mud or in burrows, and pass the winter in a deep sleep (hiberna- 

 tion)^ during which the vital fires burn low and seem well-nigh 

 extinguished. The warmth of spring re-establishes the activity 

 of the protoplasm, and in consequence animals awake from their 

 sleep and plants put forth their leaves. 



But this law is true only within certain limits. Extreme 

 heat and cold are alike inimical to life, and as the temperature 

 approaches these extremes all forms of vital action gradually or 

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

 present 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 re- 

 cent experiments show that frogs and rabbits may be chilled to 

 an unexpected degree without fatal results. 



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

 Most organisms are destroyed at the temperature of boiling 

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

 a much higher temperature without destruction (120-125 C.). 

 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. 36) of certain substances in the 

 protoplasm. Thus, if a brainless frog be gradually heated, 



