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SCIENCE 



[N. S. Vol. XXX. No. 765 



some substance with a smaller amount of 

 potential energy. Thus the weights of 

 equal masses of these substances would be 

 different. Now, experiments with pendu- 

 lums, as Newton pointed out, enable us to 

 determine with great accuracy the weights 

 of equal masses of different substances. 

 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 ex- 

 periments 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 marvel- 

 lous power of radioactivity; the discovery 

 of these came much later, and is one of 

 the most striking achievements of modern 

 physics. 



These radioactive substances are con- 

 stantly giving out large quantities of heat, 

 presumably at the expense of their poten- 

 tial energy; thus when these substances 

 reach their final non-radioactive state their 

 potential energy must be less than when 

 they were radioactive. Professor Ruther- 

 ford's measurements show that the energy 

 emitted by one gram of radium in the 

 course of its degradation to non-radioactive 

 forms is equal to the kinetic energy of a 

 mass of one thirteenth of a milligram 

 moving with the velocity of light. 



This energy, according to the rule I have 

 stated, corresponds to a mass of one thir- 

 teenth of a milligram of the ether, and thus 

 a gram of radium in its radioactive state 

 must have at least one thirteenth of a milli- 

 gram more of ether attached to it than 

 when it has been degraded into the non- 

 radioactive forms. Thus if this ether 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-radioactive 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 accu- 

 racy. 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 2,000. Lately we have 

 been using at the Cavendish Laboratory a 

 pendulum whose bob was filled with uran- 

 ium oxide. We have got good reasons for 

 supposing that uranium is a parent of 

 radium, so that the great potential energy 

 and large ethereal 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 radioactive 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 ether bound by any 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 energy. Poten- 

 tial energy is usually regarded as essen- 

 tially different from kinetic energy. Po- 

 tential energy depends on the configuration 

 of the system, and can be calculated from 

 it when we have the requisite data ; kinetic 

 energy, on the other hand, depends upon 

 the velocity of the system. According to 

 the principle of the conservation of energy 

 the one form can be converted into the 

 other at a fixed rate of exchange, so that 

 when one unit of one kind disappears a 

 unit of the other simultaneously appears. 



