mi Radioactive Change. 589 



the least possible estimate of the energy of radioactive change 

 in radium. The union of hydrogen and oxygen liberates 

 approximately 4 10 3 gram-calories per gram of water pro- 

 duced, and this reaction sets free more energy for a given 

 weight than any other chemical change known. The energy 

 of radioactive change must therefore be at least twenty- 

 thousand times, and may be a million times, as great as the 

 energy of any molecular change. 



The rate at which this store of energy is radiated, and in 

 consequence the life of a radio-element, can now be considered. 

 The order of the total quantity of energy liberated per second 

 in the form of rays from 1 gram of radium may be calcu- 

 lated from the total number of ions produced and the energy 

 required to produce an ion. In the solid salt a great pro- 

 portion of the radiation is absorbed in the material, but the 

 difficulty may be to a large extent avoided by determining 

 the number of ions produced by the radiation of the emana- 

 tion, and the proportionate amount of the total radiation of 

 radium due to the emanation. In this case most of the rays 

 are absorbed in producing ions from the air. It was experi- 

 mentally found that the maximum current due to the 

 emanation from 1 gram of radium, of activity 1000 compared 

 with uranium, in a large cylinder filled with air, was 1*65 10" 8 

 electromagnetic units. Taking: e — 2 10 -20 , the number of ions 

 produced per second =8*2 10 11 . These ions result from the 

 collision of the projected particles with the gas in their path. 

 Townsend (Phil. Mag. 1901, vol. i.), from experiments on the 

 production of ions by collision, has found that the minimum 

 energy required to produce an ion is 10 -11 ergs. Taking the 

 activity of pure radium as a million times that of uranium, 

 the total energy radiated per second by the emanation from 

 1 gram of pure radium = 8200 ergs. Ln radium compounds 

 in the solid state, this amount is about '-4 of the total energy 

 of radiation, which therefore is about 

 2 10 4 ergs per second, 

 6*3 10 11 ergs per year, 

 15,000 gram-calories per year. 

 This again is an under-estimate, for only the energy em- 

 ployed in producing ions has been considered, and this may 

 be only a small fraction of the total energy of the rays. 



Since the a. radiation of all the radio-elements is extremely 

 similar in character, it appears reasonable to assume that the 

 feebler radiations of thorium and uranium are due to these 

 elements disintegrating less rapidly than radium. The energy 

 radiated in these oases is about 10~ 6 that from radium, and 

 Phil. Mag. S. 6. Vol 5. No. 29. May L903. 2 R 



