APPENDIX 359 



potential energies, into which we suppose that the 

 energies in question have become transformed, in order 

 that we may still think of them as being subject to an 

 a priori principle of conservation. Although a particle 

 of radium continually generates heat, we do not there- 

 fore think of the first principle of energetics as being 

 invalidated, for we suppose that the energy which thus 

 appears was really potential in the atoms of radium. 

 But it was contrary to all our former experience of atoms 

 that they should contain any other energy than that of 

 their own motion, and so the further assumption was 

 made that the atom, at least the atom of the radio- 

 active substance, is really complex, and not simple, as 

 chemical theory demands. It is made up of smaller 

 particles, and possesses a definite structure. In certain 

 circumstances the atom may disintegrate, and the 

 energy which held together its particles, whether these 

 were simpler corpuscles or electrons, is given off as the 

 heat which the radio-active substance apparently 

 generates. The potential energy of the chemical atom 

 is therefore a hypothesis which has been devised in 

 order to preserve the validity of the law of conservation, 

 and the reality of this hypothesis is being tested by 

 investigation. If we accept it as true, are the deduc- 

 tions made from it justified in our experience ? That 

 is the test which must be satisfied in all the hypotheses 

 where potential energies are invented, and the potentials 

 are only real if the test is satisfactory. The golf ball 

 at rest at the top of the hill is a different entity from 

 the golf ball at rest at the bottom of the hill : it is 

 capable of developing energy, for a touch may cause it 

 to roll down the hill, when most of the energy which 

 was expended in order to drive it to the top of the hill 

 will reappear in the form of the kinetic energy of motion 

 of the ball. The atoms of hydrogen and oxygen which 



