SENSATION 



plasm has experienced considerable changes in its sense 

 of warmth through adaptation to different environments ; 

 not only the maximum and the minimum, but the opti- 

 mum (most agreeable point), is subject to very great 

 variations. This can easily be observed and followed 

 experiinentally in the phenomena of thermotaxis or 

 thermotropism — that is to say, the effect that follows 

 from a one-sided action of the heat - stimulus. The 

 organism that falls below the minimum of temperature 

 is said to be stiff with cold, while the organism that rises 

 above the maximum is stiff with heat. 



The heat-stimulus acts on inorganic as well as organic 

 bodies, like the light-stimulus. The law holds good in 

 both cases that higher temperatures increase sensation, 

 while lower ones paralyze it. There is a minimum, an 

 optimiun, and a inaximum, for many chemical and 

 physical processes in the inorganic world. As far as 

 the melting effect of water is concerned, freezing is the 

 minimum of the heat stimulus and boiling the maximum. 

 As the various chemical compounds meet in water at very 

 different temperatures, we ha.ve an optimum for many 

 substances — that is to say, a degree of warmth which is 

 most favorable to the solution of a given quantity of a 

 solid body in water. On the whole, the law holds for 

 chemical processes that they are accelerated by high 

 temperatures and retarded by low ones (like the hum^in 

 passions!); the former have a stimulating and the lat- 

 ter a benumbing effect. As the action of the various 

 chemical compounds on each other is determined by 

 the nature of the elements and their affinities, we must 

 trace the variations in their conduct towards thermic 

 stimuli to a sensation of temperature in the constituent 

 atoms; increase of temperature stimulates it, while de- 

 crease lessens or paralyzes it. Here, again, the simple 

 inorganic processes have a general resemblance to the 

 complicated vital phenomena in the organic body. 



301 



