348 



SCIENCE. 



[N. S. Vol. XXIV. No. 611. 



my letter to you of August 9 regarding radium. 



After a kind personal reference to myself, Sir 

 Oliver Lodge says, " but it is also known that " 

 Lord Kelvin's mind " lias not always submitted 

 patiently to the task of assimilating the work of 

 others by the process of reading, and our hope has 

 been that before long he would find time and 

 inclination to look into the evidence more fully." 

 I am quite sure my old friend Lodge could not 

 wilfully be unjust to me, but I do not think he 

 knows how carefully and appreciatively I have 

 . done all I could by reading, and by personal inter- 

 course with many of the chief workers in the 

 field, to learn experimental results and theoretical 

 deductions regarding radioactivity, ever since its 

 discovery ten years ago by Henri Becquerel. I 

 scarcely think any other person has spent more 

 hours in reading the first and second editions of 

 Rutherford's ' Radioactivity ' than I have. 



Both Lodge and Strutt refer to the slowness of 

 the extraction of helium from radium, in the 

 process hitherto followed; and there seems to be 

 good reason to believe that this slowness is essen- 

 tial. Strutt says, " If all helium has been re- 

 moved from a sample of radium, it is found that, 

 after an interval, a further supply can be ex- 

 tracted." The last clause shows that the initial 

 ' if ' was wrong ; and that some helium remained 

 in the modified sample of radium. This view is 

 thoroughly in accordance with Rutherford's second 

 edition (1905), p. 284, in which it is suggested 

 that ' uranium, thorium and radivun are in reality 

 compounds of helium ' ; and, more particiilarly, 

 that radium (atomic weight 225) may be a com- 

 pound of four atoms of helium (atomic weight 

 4X5) and one atom of lead (atomic weight 205). 



As to the suggestion, made first, I believe, by 

 Sir George Darwin about three years ago, and 

 favorably received by Rutherford, Strutt and 

 others, that our present underground heat, and 

 sun-light and sun-heat are due to radium, I must, 

 for the present, limit myself to two sentences: 

 (1) Radium alone is quite insufficient, its dura- 

 tion as a source of energy being estimated by 

 Rutherford (2d edition, p. 458) as not more than 

 a few thousand years ( ' the average life of radium 

 is 1,800 years'). (2) The suggestion that in 

 uranium, thorium, actinium and other matter 

 capable of being ' transformed slowly into 

 radium,' we have at least a million times as great. 

 a store of energy as we may think we have in 

 radium, practically available for sun-heat and 

 underground heat, is not validly supported by a:ny 

 experimental evidence hitherto published. 



Kelvin. 



Your leading article (August 18) and recent 

 correspondence show that an emphatic statement 

 is needed to make clear to your lay readers: 



(1) That radium is not an ordinary chemical 

 compound. 



(2) That it does disintegrate with explosive 

 violence. 



(3) That it is present in sufficient quantity to 

 account for the heat of the earth. 



(4) That its disintegration is not altered by 

 any known physical conditions. 



Some experimental results obtained by me last 

 spring at Montreal, under the guidance of Pro- 

 fessor Rutherford^ are in good agreement with 

 those found independently and by a different 

 method by the Hon. R. J. Strutt. The equivalent 

 amount of radium present in the upper layers of 

 the earth was determined by the penetrating 

 radiation resulting from the active matter. An 

 account of this work has been communicated to 

 the Philoso'phical Magazine and is in the printer's 

 hands. 



It is certain that radium does occur in sufficient 

 quantity, and that its heat energy is of sufficient 

 magnitude, to account for the existing tempera- 

 ture-gradients of the earth. Until we can con- 

 duct experiments in the interior of the earth, the 

 radium theory is entitled to at least as much 

 weight as the gravitation theory. 



The recent discussion in your columns has re- 

 sulted in a goodly crop of errors, written by 

 those who have contented themselves with read- 

 ing rather than with research work. I believe 

 that no actual worker at the subject dissents from 

 the generg^l conclusions contained in Rutherford's 

 ' Radioactivity.' 



For example, it has been implied that radium is 

 merely an ordinary compound of lead ( ? ) and 

 helium. But no ordinary chemical compound pro- 

 jects a particle, having the mass of a helium 

 atom, with a velocity almost of the order of the 

 velocity of light. 



It has been stated that the disintegration of 

 I'adium occurs too slowly to be detected by direct 

 observation, and that is true. But some of the 

 other active bodies, such as the subsequent 

 products of radium, disintegrate with no less 

 violence in a few minutes. It has been suggested 

 that the disintegration may be retarded by pres- 

 sure or by lack of concentration; but all the evi- 

 dence — and there is much — indicates that the 

 process of disintegration can neither be acceler- 

 ated nor retarded by temperature, pressure, con- 

 centration, or other physical changes. 



