ELEMENTARY VITAL PHENOMENA 211 



itself is always the same, and that the different forms of its appear- 

 ance are merely cloaks, which may be exchanged according to the 

 conditions at the moment. 



Just as we speak of different forms of energy, we can distinguish 

 in the single form two different modifications, according as the energy 

 actually produces motion or only has potentially the capacity of 

 putting into action under proper conditions. Physicists term these 

 two modifications kinetic energy (also actual energy, or energy of 

 motion) and potential energy (energy of position). The energy of 

 gravitation, e.g., is kinetic when it draws a stone to the earth at 

 the moment when the stone is set free ; it is potential so long 

 as the stone is fixed above the earth's surface. Likewise, chemical 

 energy is kinetic when it brings two atoms to each other ; but it is 

 potential when an atom has no other one in its vicinity that it can 

 attract. Kinetic energy passes over constantly into potential energy 

 and vice versa. 



The law of the conservation of energy, therefore, controls all that 

 happens in nature; it is the fundamental law of energetics. 

 According to it, as has already been seen, energy in the world 

 never originates or disappears ; the sum of energy in the world is 

 constant, just as the law of the conservation of matter expresses 

 the same constancy in the quantity of matter. Where a certain 

 quantity of energy seems to originate or disappear, in reality it 

 simply goes over into another form or modification. If, e.g., an 

 electric current be passed through a vessel containing water, the 

 electrical energy seems to be lost. But in reality it does not 

 go out of existence, for it has been seen that the molecules of 

 the water are decomposed into their hydrogen and oxygen atoms, 

 and these accumulate in a gaseous state upon the two poles of the 

 electrical conductors. Hence the electric current has performed 

 work and has separated the atoms of the molecules of water from 

 one another. But the atoms of hydrogen and oxygen set free 

 have a chemical affinity for one another ; hence in the experiment 

 the kinetic energy of the electric current has simply been trans- 

 formed into the potential energy of chemical affinity. If, therefore, 

 the separate atoms of hydrogen and oxygen be brought again into 

 union under proper conditions, the chemical potential passes over 

 again into kinetic energy, and a certain quantity of heat is liber- 

 ated thereby. This heat can be transformed again into electricity 

 in a thermo-electric apparatus, and, if the technical difficulties 

 would allow the whole experiment to be carried out with 

 sufficient exactness, it would be found that the same quan- 

 tity of electricity has again been obtained as was consumed 

 previously in the splitting-up of the water. During all trans- 

 formations the original quantity of energy remains the same. In 

 order to have a unit for the measurement of any quantity of 

 energy, physicists have chosen, in accordance with Joule's 



p 2 



