INTRODUCTION. 13 



ture. Vital force, or vitality, had been thought of 

 as something distinct in itself ; and that there was 

 any measurable relation between the powers of 

 the living organism and the forces of heat and 

 chemical affinity was of course unthinkable be- 

 fore the formulation of the doctrine of the corre- 

 lation of forces. But as soon as that doctrine was 

 understood it began to appear at once that, to a 

 certain extent at least, the living body might be 

 compared to a machine whose function is simply 

 to convert one kind of energy into another. A 

 steam engine is fed with fuel. In that fuel is a 

 store of energy deposited there perhaps centuries 

 ago. The rays of the sun, shining on the world 

 in earlier ages, were seized upon by the growing 

 plants and stored away in a potential form in the 

 wood which later became coal. This coal is placed 

 in the furnace of the steam engine and is broken 

 to pieces so that it can no longer hold its store 

 of energy, which is at once liberated in its active 

 form as heat. The engine then takes the energy 

 thus liberated, and as a result of its peculiar mech- 

 anism converts it into the motion of its great fly- 

 wheel. With this notion clearly in mind the ques- 

 tion forces itself to the front whether the same 

 facts are not true of the living animal organism. 

 It, too, is fed with food containing a store of en- 

 ergy ; and should we not regard it, like the steam 

 engine, simply a machine for converting this po- 

 tential energy into motion, heat, or some other 

 active form ? This problem of the correlation 

 of vital and physical forces is inevitably forced 

 upon us with the doctrine of the correlation of 

 forces. Plainly, however, such questions were 

 inconceivable before about the middle of the 

 present century. 



