Effect of Temperature. 125 



CHAPTER XIX. 



EFFECT OF TEMPERATURE. 



The effect of heat and cold on organisms was alluded to in connec- 

 tion with what was said on periodicity. The periodic recurrence of 

 changes in temperature induce periodic changes in organic habits of 

 both vegetables and animals. Some further inquiry will show the 

 proximate causes. 1 Temperature expresses the degree of molecular vi- 

 bration going on in the constituent particles of a body. The higher the 

 degree of temperature the greater isthe molecular activity of the parti- 

 cles, and the further apart are the molecules separated from each other. 

 Hence a heated body expands, and the attraction of the molecules of 

 which it is composed, for each other, is either lost or neutralized by an 

 opposing force. The molecules of all bodies require to be thus shaken 

 from each other's embraces before they can be brought into new combi- 

 nations with other bodies. The degree of temperature at which the 

 chemistry of the vital functions can be carried on depends on the com- 

 plexity of the molecular constitution of the organs in which the function 

 is performed, and which differs greatly in different organisms, and 

 within bounds largely on habit, as between those organisms which ap- 

 pear to be identical in structure. Again some of the functions neces- 

 sary to a continuance and reproduction of vitality may* be carried on at 

 a temperature that causes a suspension of others. The general limits 

 of life are between the freezing and the boiling points of water, and 

 100 centigrade or 32 and 212 Fahrenheit. But no organism can carry 

 on all its functions very near either of those limits. There are certain 

 organic germs, however, that it requires more than boiling heat to 

 destroy, and the eggs of some fish and insects, crustaceans, bryozoans 

 and sponges, are not destroyed by a temperature below freezing. But 

 in both cases probably the temperature of the organism is not changed 

 quite to that of its medium. 



Both animal and vegetable protoplasm is killed at from 40 to 50 C. , 

 but some crustaceans and the larvae of insects and some algae live in 

 water from 50 to 60 C. Infusoria possess in the interior of their bit of 

 protoplasm a contractile vesicle which contracts and dilates with some reg- 

 ularity at its normal temperature, which is between 15 and 30 C. At 

 15 the pulsations were observed to be fifteen per minute, and the num- 

 ber increased with the temperature. But at 10 the pulsations were re- 

 duced to from 2 to 9 per minute, the different species of these little 



1 Many of the facts mentioned in this chapter and the next I have culled from Karl 

 Semper's "Animal Life." 



