ON PHYSICO-CHEMICAL METHODS AND PROBLEMS. 7 



phenomena in spite of most extensive utilization of the 

 MAYER-JOULE law. 



The energetic principle suffices equally little in bio- 

 logical questions, and we must regard the attempt of an 

 excellent investigator to define general physiology as 

 the energetics of life phenomena as not sufficiently 

 comprehensive. Our law determines only the energy value 

 corresponding with the changes that take place in living 

 matter; the fundamental question of their nature remains 

 entirely unanswered. 



A picture of natural phenomena which shows only 

 differences in energy is as incomplete as a photograph 

 which shows only differences in light and shade. 



Upon the second assertion of OSTWALD that we react 

 only in proportion ' to differences in energy we must also 

 place certain limitations. 



What we designate as external stimuli are changes 

 which also are connected with variations in energy. The 

 important point in our question is whether differences 

 in energy determine quantitatively the excitation value 

 of a stimulus. If this is true, then electrical, thermal, or 

 mechanical stimuli having the same energy value ought 

 to possess the same excitation value. Things are by 

 no means as simple as this, however. We do not per- 

 ceive the energy communicated to our sense-organs 

 directly. What we perceive are only changes in the 

 state of our sensory nerves, a fact recognized by DESCARTES 

 in his day and, as pointed out by JOHANNES MULLER, 

 suggested even by PLATO. 



When MULLER postulates in the famous laws which 

 bear his name qualitatively different changes in state in 

 each variety of sensory nerve, changes which for different 



