760 THE POPULAR SCIENCE MONTHLY. 



the extremities. The emotions, too, left a record. When only a stu- 

 dent came into the room, little or no effect appeared in the curve ; 

 but, when Professor Ludwig himself came in, the arteries in the arm 

 of the person in the apparatus contracted quite as strongly as upon a 

 very decided electrical stimulation. 



In an address of the retiring President of this Association, deliv- 

 ered but a few years ago, I find this sentence : " Thought can not be 

 a physical force, because thought admits of no measure." In the light 

 of the rapid advances lately made in investigating mental action, we 

 see that in two directions at least, in its rate of action and of its rela- 

 tive energy, we may already measure thought, as we measure any 

 other form of energy, by the effects it produces. 



Passing now to the consideration of the general question of the 

 transformation of energy which is effected by living beings, attention 

 may be called to one or two points in general physics, as bearing upon 

 its solution. The great law of the dissipation of energy, as modified 

 by Thomson from the statement of Clausius, is thus stated : " The 

 entropy of the universe tends to zero." In other words, the energy of 

 the universe available for transmutation is approaching extinction. 

 This conclusion is based upon the fact that while every form of energy 

 can be completely converted into heat, heat can not be completely 

 converted into other forms of energy, nor these into each other. 

 Hence it arises that energy is being gradually dissipated as heat. 

 Moreover, since transformation can only result when heat passes from 

 a higher to a lower temperature, it follows that, when that perfect 

 equilibrium of temperature is reached toward which events are tend- 

 ing, there can be no other energy than heat, and this absolutely 

 inconvertible, irrecoverable. To apply this law to the present case, 

 the muscle, for example, is a machine for transforming the energy of 

 food into work. Since, consequently, this conversion is not complete, 

 it follows that heat must appear as a necessary result of muscular 

 action. The heat of animal life, consequently, is not heat especially 

 provided ; it is simply the heat which inevitably results from an in- 

 complete conversion of energy. 



Again, the form of chemical action thus far assumed by physiolo- 

 gists to account for the energy of the living animal has been combus- 

 tion. But the science of thermo-chemistry, as developed in late years 

 by Berthelot and Thomson, has proved that direct union of chemical 

 substances may not only not evolve heat, but may actually absorb it. 

 It appears, too, that thermal changes accompany all forms of chemical 

 change, those of decomposition and exchange as well as those of syn- 

 thesis. The animal absorbs highly complex substances as food, ca- 

 pable of innumerable stages of retrogressive metamorphosis before 

 elimination. In each of these stages heat is evolved, being the energy 

 successively stored up by the plant when it repeated these stages in 

 the inverse order. 



