254 



NA TURE 



[August 26, 1909 



equal the electrostatic potential energy of the particle. 

 This. result can be extended, to any electrified system, and 

 it can be shown that such a: system binds a mass of the 

 aether proportional to its potential energy. Thus a part 

 of the mass of any system is proportional to the potential 

 energy of the system. 



The question now arises, Does this part of the mass 

 add anything to the weight of the body? If the a;ther 

 were not subject to gravitational attraction it certainly 

 would not ; and even if the rether were ponderable, we 

 might expect that as the mass is swimming in a sea of 

 aether it would not increase the weight of the body to 

 which it is attached. But if it does not, then a body 

 with a large amount of potential energy may have an 

 appreciable amount of its mass in a form which does not 

 increase its weight, and thus the weight of a given mass 

 of it may be less than that of an equal mass of some 

 substance with a smaller amount of potential energy. 

 Thus the weights of equal masses of these substances 

 would be different. Now, experiments with pendulums, as 

 Newton pointed out, enable us to determine with great 

 accuracy the weights of equal masses of different sub 

 stances. Newton himself made experiments of this kind, 

 and found that the weights of equal masses were the same 

 for all the materials he tried. Bessel, in 1830, made some 

 experiments on this subject which are still the most 

 accurate we possess, and he showed that the weights of 

 equal masses of lead, silver, iron, brass did not differ by 

 as much as one part in 60,000. 



The substances tried by Newton and Bessel did not, 

 however, include any of those substances which possess 

 the marvellous power of radio-activity ; the discovery of 

 these came much later, and is one of the most strilcing 

 achievements of modern physics. 



These radio-active substances are constantly giving out 

 large quantities of heat, presumably at the expense of 

 their potential energy ; thus when these substances reach 

 their final non-radio-active state their potential energy 

 must be less than when they were radio-active. Prof. 

 Rutherford's measurements show that the energy emitted 

 by one gram of radium in the course of its degradation 

 to non-radio-active forms is equal to the kinetic energy 

 of a mass of i/i3th of a milligram moving with the 

 velocity of light. 



This energy, according to the rule I have stated, corre- 

 sponds to a mass of i/i3th of a milligram of the aether, 

 and thus a gram of radium in its radio-active state must 

 have at least i/i3th of a milligram more of aether attached 

 to it than when it has been degraded into the non-radio- 

 active forms. Thus if this ^ther does not increase the 

 weight of the radium, the ratio of mass to weight for 

 radium would be greater by about one part in 13,000 than 

 for its non-radio-active products. 



I attempted several years ago to find the ratio of mass 

 to weight for radium by swinging a little pendulum, the 

 bob of which was made of radium. I had only a small 

 quantity of radium, and was not, therefore, able to attain 

 any great accuracy. I found that the difference, if any, 

 in the ratio of the mass to weight between radium and 

 other substances was not more than one part in 2000. 

 Lately we have been using at the Cavendish Laboratory 

 a pendulum the bob of which was filled with uranium 

 oxide. We have got good reasons for supposing that 

 uranium is .a parent of radium, so that the great potential 

 energy and large sethereal mass possessed by the radium 

 will be also in the uranium ; the experiments are not yet 

 completed. It is, perhaps, expecting almost too much to 

 hope that the radio-active substances may add to the great 

 services they have already done to science by furnishing 

 the first case in which there is some differentiation in the 

 action of gravity. 



The mass of aether bound by anv system is such that if 

 it were to move with the velocity of light its kinetic 

 energy would be equal to the potential energy of the 

 system. This result suggests a new view of the nature 

 of potential energv. Potential energy is usually regarded 

 as essentially different from kinetic energy. Potential 

 energy depends on the configuration of the svsfem, and 

 can be calculated from it when we have the requisite data ; 

 kinetic energy, on the other hand, deoends upon the velocity 

 of the system. According to the principle of the conserva- 



NO. 2078, VOL. Si] 



tion of energy the one .form can be converted into the other 

 at a fixed rate of exchange, so that when one unit of one 

 Icind disappears a unit of the other simultaneously .appears. 



Now in many cases this rule is all that we require to 

 calculate the behaviour of the system, and the conception 

 of potential energy, is of the utmost value in making the 

 knowledge derived from experiment and observation avail- 

 able for mathematical calculation. It must, however, I 

 think, be admitted that from the purely philosophical point 

 of view it is open to serious objection. It violates, for 

 example, the principle of continuity. When a thing 

 changes from a state A to a different state B, the prin- 

 ciple of, continuity requires that it must pass through a 

 number of states intermediate between A and B, so that 

 the transition is made gradually, and not abruptly. Now, 

 when kinetic energy changes into potential, although there 

 is no discontinuity in the quantity of the energy, there 

 is in its quality, for we do not recognise 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 cliange ^er salhtni from the j 

 kinetic to the potential form. In the case of the transition 

 of kinetic energy 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 

 Ivinetic into thermal energy seemed discontinuous ; but 

 now we know that this energy is the kinetic energy of 

 the molecules 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 energv as the kinetic energy of 

 portions of the a-ther attached to the system, then all 

 energy is kinetic energy, due to the motion of matter or 

 of portions of aether attached to the matter. I showed, 

 many years ago, in my " Applications of Dynamics to 

 Physics and Chemistry," that we could imitate the effects 

 of the potential energy of a system by means of the kinetic 

 energy of invisible systems connected in an appropriate 

 manner with tlie 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 universe is the 

 luminiferous a;ther, that portions of the fether in rapid 

 motion are connected with the visible systems, and that 

 their kinetic energy is the potential energy of the systems. 



We may thus regard the aether as a bank in which 

 we may deposit energy and withdraw it at our convenience. 

 The mass of the fether attached to the system will change 

 as the potential energy changes, and thus the mass of a 

 system the potential energy of which is changing cannot 

 be constant; the fluctuations in mass under ordinary con- 

 ditions are, however, so small that they cannot be detected 

 bv 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 matter, 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 jether 

 gripped by the material universe . to become smaller and 

 smaller; the rate at which it would diminish would, how- 

 ever, 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 incandescent body 

 like the sun involves a constant 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 per year is 

 only one part in 20 billionths (i in 2X10") of the mass 

 of the sun, and as this diminution in mass is not neces- 

 sarily accompanied by any decrease in its gravitational 

 attraction, we cannot expect to be able to get any evidence 

 of this effect. 



As our knowledge of the properties of light has pro- 

 gressed, we have been driven to recognise that the aether, 

 when transmitting light, possesses properties which, before 

 the introduction of the electromagnetic theory, would have 

 been thought to be peculiar to an emission theory of light 

 and to be fatal to the theory that light consists of un- 

 dulations. 



