CHAPTER VIII 



ACTION OF HEAT UPON PROTOPLASM 



In this chapter it is proposed to consider (I) briefly, the 

 nature of heat and the general methods of its application ; 

 (II) the action of heat upon the general functions of organ- 

 isms ; (III) the temperature-limits of life ; (IV) the accli- 

 matization of organisms to extreme temperature, and (V) the 

 determination of the direction of locomotion by heat — ther- 

 motaxis. 



§ 1. Nature of Heat and the General Methods op 

 ITS Application 



Heat is believed to be due to the vibrations of the molecules 

 of bodies. In any heated solid, fluid, or gaseous mass the 

 molecules are in constant motion. When the temperature 

 is increased, the motion is increased, and the impacts of the 

 flying molecules become more frequent. If a vessel containing 

 water is brought into contact with warmer air or warmer fluid, 

 its molecules fly faster, its temperature is raised. As the 

 motion of the molecules in the walls of the vessel increases, 

 the increased motion is transmitted to the contained water, 

 and finally to the objects in the water. Thus the motion of 

 the molecules of an organism in the water is increased with the 

 increase of the temperature of the water. Heat, as so-called 

 radiant heat, is transmitted through space in straight lines, and 

 follows all the laws of light, into which it passes when the rate 

 of wave vibrations becomes rapid enough to affect the retina. 

 The chief effects of radiant heat were considered in the last 

 chapter. 



Heat is an important element in all chemical processes. The 

 state of cohesion — solid, liquid, gaseous — and the ease with 



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