August 27, 1909] 



SCIENCE 



271 



Now in many cases this rule is all that 

 we require to calculate the behavior of the 

 system, and the conception of potential 

 energy is of the utmost value in making the 

 knowledge derived from experiment and 

 observation available for mathematical cal- 

 culation. It must, however, I think, be 

 admitted that from the purely philosoph- 

 ical point of view it is open to serious ob- 

 jection. It violates, for example, the prin- 

 ciple of continuity. When a thing changes 

 from a state A to a. different state B, the 

 principle of continuity requires that it 

 must pass through a number of states inter- 

 mediate between A and B, so that the 

 transition is made gradually, and not ab- 

 ruptly. Now, when kinetic energy changes 

 into potential, although there is no discon- 

 tinuity in the quantity of the energy, there 

 is in its quality, for we do not recognize 

 any kind of energy intermediate between 

 that due to the motion and that due to the 

 position of the system, and some portions 

 of energy are supposed to change per sal- 

 tum from the kinetic to the potential form. 

 In the case of the transition of kinetic en- 

 ergy into heat energy in a gas, the discon- 

 tinuity has disappeared with a fuller 

 knowledge of what the heat energy in a 

 gas is due to. When we were ignorant of 

 the nature of this energy, the transition 

 from kinetic into thermal energy seemed 

 discontinuous; but now we know that this 

 energy is the kinetic energy of the mole- 

 cules of which the gas is composed, so that 

 there is no change in the type of energy 

 when the kinetic energy of visible motion 

 is transformed into the thermal energy of 

 a gas— it is just the transference of kinetic 

 energy from one body to another. 



If we regard potential energy as the 

 kinetic energy of portions of the ether at- 

 tached to the system, then all energy is 

 kinetic energy, due to the motion of matter 

 or of portions of ether attached to the mat- 



ter. I showed, many years ago, in my 

 "Applications of Dynamics to Physics and 

 Chemistry," that we could imitate the ef- 

 fects of the potential energy of a system by 

 means of the kinetic energy of invisible 

 systems connected in an appropriate man- 

 ner with the main system, and that the 

 potential energy of the visible universe 

 may in reality be the kinetic energy of an 

 invisible one connected up with it. We 

 naturally suppose that this invisible uni- 

 verse is the luminiferous ether, that por- 

 tions of the ether in rapid motion are con- 

 nected with the visible systems, and that 

 their kinetic energy is the potential energy 

 of the systems. 



We may thus regard the ether as a bank 

 in which we may deposit energy and with- 

 draw it at our convenience. The mass of 

 the ether attached to the system will change 

 as the potential energy changes, and thus 

 the mass of a system whose potential en- 

 ergy is changing can not be constant; the 

 fluctuations in mass under ordinary condi- 

 tions are, however, so small that they can 

 not be detected by any means at present at 

 our disposal. Inasmuch as the various 

 forms of potential energy are continually 

 being changed into heat energy, which is 

 the kinetic energy of the molecules of mat- 

 ter, there is a constant tendency for the 

 mass of a system such as the earth or the 

 sun to diminish, and thus as time goes on 

 for the mass of ether gripped by the ma- 

 terial universe to become smaller and 

 smaller; the rate at which it would dim- 

 inish would, however, get slower as time 

 went on, and there is no reason to think 

 that it would ever get below a very large 

 value. 



Radiation of light and heat from an in- 

 candescent body like the sun involves a con- 

 stant loss of mass by the body. Each unit 

 of energy radiated carries off its quota of 

 mass, but as the mass ejected from the sun 



